What harmful effects is the second benefit of the Shield Master feat useful against?

Question

One of the benefits of the Shield Master feat in 5e (PHB, p. 170) reads:

If you aren't incapacitated, you can add your shield's AC bonus to any Dexterity saving throw you make against a spell or other harmful effect that targets only you.

Though it's pretty clear which spells it's useful against (spoiler: not many), I'm still puzzled about what other harmful effects it can be helpful to protect from.

If I read the rule literally, it turns out, that I can't benefit from this bonus against an avalanche (because it's an AoE), but I can use it against getting into a hunting trap, that honestly doesn't have anything to do with whether I have a shield or not, which seems absolutely counter-intuitive.

My question basically consists of two parts:

1. Do I understand RAW correctly? (For instance, maybe there is a narrow definition of "harmful effects" somewhere, that I don't know about.)
2. What are the guidelines (preferably with examples) to determine, whether Shield Master feat gives a bonus against a certain harmful effect.

Answer 1

Harmful Effects Defined

The term "Harmful Effects" is not actually defined by 5E, even though they use the term in several places. The closest we get to a definition is this:

Damage Types

Different attacks, damaging spells, and other harmful effects deal different types of damage.

so we go with the plain English definition. Harmful means:

causing or capable of causing harm

And Harm is defined as:

physical injury or mental damage;

Thus, we can conclude that "Harmful Effects" speaks specifically of things that can damage you. In short, by a strict usage of the English Language--that Shield Master feature applies against effects that...

1. Only target you
2. Require a Dex Save
3. Deal Damage

This meshes with the Damage Types statement linked above, as it talks about Harmful Effects dealing types of damage.

Specific Examples

I'm pulling all of these from Class Features, as they are the easiest to search

Feature works against

These class features meet all three criteria: They only target one person, require a Dex Save, and deal damage.

Storm Herald Barbarian's Storm Aura, Sea Option

When this effect is activated, you can choose one other creature you can see in your aura. The target must make a Dexterity saving throw. The target takes 1d6 lightning damage on a failed save, or half as much damage on a successful one.

Tempest Cleric's Wrath of the Storm

Also at 1st level, you can thunderously rebuke attackers. When a creature within 5 feet of you that you can see hits you with an attack, you can use your reaction to cause the creature to make a Dexterity saving throw. The creature takes 2d8 lightning or thunder damage (your choice) on a failed saving throw, and half as much damage on a successful one.

Arcane Archer's Seeking Arrow

[...]If the target is within the weapon’s range and there is a path large enough for the arrow to travel to the target, the target must make a Dexterity saving throw. Otherwise, the arrow disappears after traveling as far as it can. On a failed save, the target takes damage as if it were hit by the arrow, plus an extra 1d6 force damage, and you learn the target’s current location. On a successful save, the target takes half as much damage, and you don’t learn its location.

Monk, Way of Four elements: Water Whip

You can spend 2 ki points as an action to create a whip of water that shoves and pulls a creature to unbalance it. A creature that you can see that is within 30 feet of you must make a Dexterity saving throw. On a failed save, the creature takes 3d10 bludgeoning damage, plus an extra 1d10 bludgeoning damage for each additional ki point you spend[...]

Feature does not work against

In both of these examples, you are making a Dexterity Save to evade an effect that doesn't deal damage. Thus, the Shield Master feature does not aid you here.

Way of the Open Hand: Open Hand Technique

Whenever you hit a creature with one of the attacks granted by your Flurry of Blows [...] It must succeed on a Dexterity saving throw or be knocked prone.

Paladin Oath of the Ancients, Nature's Wrath

You can use your Channel Divinity to invoke primeval forces to ensnare a foe. As an action, you can cause spectral vines to spring up and reach for a creature within 10 feet of you that you can see. The creature must succeed on a Strength or Dexterity saving throw (its choice) or be restrained.

Answer 2

The flaming sphere spell's ramming effect

Beyond the spell effects listed in the linked answer, there is a feature of the flaming sphere spell that the Shield Master feat's defensive feature can be used against.

As a bonus action, you can move the sphere up to 30 feet. If you ram the sphere into a creature, that creature must make the saving throw against the sphere’s damage, and the sphere stops moving this turn.

If an enemy spellcaster with a flaming sphere uses this to do damage to a PC with that feat on their (the enemy's) turn, the shield master saving throw bonus applies.(Thanks @CrusaderJ for raising this point)

A hunting trap

Hunting Trap.
... this trap forms a saw-toothed steel ring that snaps shut when a creature steps on a pressure plate in the center. The trap is affixed by a heavy chain to an immobile object, such as a tree or a spike driven into the ground. A creature that steps on the plate must succeed on a DC 13 Dexterity saving throw or take 1d4 piercing damage and stop moving. (p. 51, Basic Rules)

The "a creature" targets "only you" if you are the PC with the Shield Master feat and you step into a trap. The bonus to the Dexterity save may help prevent needing to chew one's leg off to get out of the trap. :)

Other traps with a Dexterity save to avoid/reduce damage

The Pit Trap in Xanathar's Guide to Everything (p. 115) calls for a creature to make a Dexterity save. The Scything Trap, if the creature is alone in triggering the trap, should see the Shield Master bonus aid in making the save since at that point "only the creature" is targeted. But with multiple party members subject to the trap, the traps effect looks more like "AoE" and I can see a DM ruling that Shield Master won't help.

A number of the complex traps in Xanathar's Guide to Everything may be "single targeted" enough for Shield Master to boost the Dexterity save, but some of the effects come off as more "AoE" in nature. These will require a DM ruling.

In contrast, the Barbarian's second level class feature (Danger Sense) will be effective in avoiding or reducing damage for most of these traps, since it is worded less specifically than Shield Master is.