Why would I put an ASI into Con instead of taking the Tough feat?

Question

I know that raising your Constitution modifier increases your maximum health. However, the Tough feat also does this, but gives twice as much health. What other benefits does a +2 to Con give, and when does that make it a better choice?

Answer 1

Constitution is not only good for hit points

You will have to weigh how important all of the following are for your character, compared to the 1 extra maximum hit point per level.

The PHB points out several uses for Constitution (p. 177):

The DM might call for a Constitution check when you try to accomplish tasks like the following:

• Hold your breath
• March or labor for hours without rest
• Go without sleep
• Survive without food or water
• Quaff an entire stein of ale in one go

Some of these, like starvation, tiring or suffocation, can have severe mechanical impact as they cause exhaustion, which can outright kill you, no matter how many hit points you have. A higher stat bonus will also improve your Constitution saving throw, which is important for

• Resisting spells with constitution saves like shatter, flesh to stone, or finger of death
• Resisting the effects of poison or disease
• Resisting other harmful effects such as a monk's stunning strike
• Resisting suffocation and starvation
• Being able to keep up forced marches
• Maintaining Concentration on a spell when damaged

Especially for spellcasters, the last one can make a big difference in a fight, for example if you polymorphed an extremely dangerous opponent into a frog, or are flying with the fly spell at high altitudes.

Several mechanics reward higher Constitution beyond just the saves. For example,

• Starvation: you can go 3 + your Constitution modifier days without sustenance before you have to start rolling saves for exhaustion

• Suffocation: you can hold your breath for 1 + your Constitution modifier minutes before suffocating, and then can only survive for a number of rounds equal to your Constitution modifier (minimum 1), before dropping to 0 hits and dying

• Regaining hits: When you roll hit dice to regain hit points, you add your Constitution modifier to those rolls

• Recuperating in downtime: you can make a Constitution check to end one effect on you that prevents you from regaining hit points, or to, for the next 24 hours, gain advantage on saving throws against one disease or poison affecting you

• Chases, a DM option, where you are only allowed to dash Constitution bonus +3 times before needing to make Constitution checks or take exhaustion (Thanks, @AnnaAG!)

• As a barbarian, you get to add your Constitution to your AC, if you don't wear armor, and from level 11 on, can make a Con save to drop to 1 hit instead of 0 in rage

• For Dragonborn, the Constitution modifier counts towards the saving throw DC for their breath weapon

• Champion fighters from 18th level regain 5 + their Constitution modifier hit points at the start of each turn if they have less then half their hit points left

Outside of the core rules

• Going without sleep in the form of long rests without exhaustion (XGtE)
• Pit Fighting to earn money (XGtE)
• Beast barbarians level 10 Infectious Fury uses Constitution for its saving throw, their Call of the Hunt provides benefits like temp HP based on allies up to the number of your Constitution bonus,
• Wild Magic barbarians use Constitution for the saving throws their Wild Surges cause
• Rune Knight fighters use Constitution for the saving throws against their rune effects (Thank you, @ObliviousSage)
• Cavalier fighters can use Warding Maneuver Con bonus times (minimum 1) per long rest

Answer 2

HP are just one thing that Constitution affects

You are correct that your Constitution affects hp, and that the Tough feat affects your hp twice as much as raising your Constitution modifier does. However, your hp are not the only thing that is affected by your Con mod. So there is the tradeoff between ASI and feat - do you want a smaller increase for many things in general, or do you want a larger increase for one thing in particular?

As MannerPots says, your Con mod also affects your Con saves. Constitution apparently is the second-most targeted ability for spells that permit a save and far-and-away the most common ability for monster effects that permit a save (because so many monsters have poison). And as MannerPots says, Con saves are particular relevant to casters for maintaining concentration. They are also called for when you make a forced march, go without water, attempt downtime recuperation, or use a barbarian's relentless rage.

The Hobgoblin reminds us that if you are a Dragonborn, your Con mod affects the strength (DC) of your breath weapon.

Your Con mod also affects your Con checks. The PHB gives some examples of these as when you might "try to accomplish tasks like the following: Hold your breath, March or labor for hours without rest, Go without sleep, Survive without food or water, Quaff an entire stein of ale in one go"¹ (and, as AnnaAG adds, the DMG also asks for Con checks in Chases). So you know, you might be Tough enough to take more damage, but can you drink alcohol really fast?

Your Con mod might affect other things like a Barbarian's Unarmored defense, a Champion's Survival feature, how long you can last without suffocating, and the Durable Feat.

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¹ Although this list is a little strange in that except for the ale-quaffing, each of those other cases is covered elsewhere in the rules by Con saves or a flat comparison of Con mod, not Con checks.

Answer 3

Improving your Con by 2 also gives you +1 to constitution saving throws. These are commonly used to resist many spells, as well as diseases, poisons, and conditions imparted by some monster attacks.

This is even more relevant for spellcasters since concentration checks (saves made to maintain concentration on a spell after taking damage) are a type of constitution saving throw.