How can a ranger's Hail of Thorns ever do 6d10 damage?

Question

If I'm not mistaken (and I triple checked), a Ranger has spell slots up to 5th level (2 spell slots). Hail of Thorns is a 1st level spell, and if cast at a higher level, it gets an extra 1d10 per spell slot level above 1st, so a 2nd level spell slot does a total of 2d10 dmg, a 3rd level slot does 3d10 dmg, and so on. This spell has a limit up to 6d10 dmg, so how can I cast it to achieve that 6d10 dmg? I mean, a 5th level spell slot would do 5d10 dmg, so where does the extra 1d10 come from? Am I missing something?

Hail of Thorns

1st-level conjuration

Casting Time: 1 bonus action
Range: Self
Components: V
Duration: Concentration, up to 1 minute

The next time you hit a creature with a ranged weapon attack before the spell ends, this spell creates a rain of thorns that sprouts from your ranged weapon or ammunition. In addition to the normal effect of the attack, the target of the attack and each creature within 5 feet of it must make a Dexterity saving throw. A creature takes 1d10 piercing damage on a failed save, or half as much damage on a successful one.

At Higher Levels. If you cast this spell using a spell slot of 2nd level or higher, the damage increases by 1d10 for each slot level above 1st (to a maximum of 6d10).

Answer 1

The 6d10 maximum damage is not relevant for an usual ranger, but is brought into play when casting the spell at higher levels than five, which can happen...

By being a Bard

The Bard has spell slots all way to ninth level, and the class feature magical secrets allows them to learn spells of any class. A Bard could cast the spell with a sixth-level slot or above, dealing 6d10 damage.

By multiclassing into a spellcaster

If a Ranger takes the spell early, they can multiclass into Wizard, Sorcerer, Bard, Cleric or Druid and eventually gain access to sixth-level and higher level slots and use them to cast the spell at higher-than-usual levels.

By casting Wish to duplicate the effects of Hail of Thorns

A high-level caster can use the spell Wish to duplicate the effects of any lower-level spell, including Hail of Thorns. In this case, it'd be cast as a ninth level spell as described here.

Answer 2

On top of kviiri's great answer, the DM might hand out magic items or magical effects that increase your spell slot level (temporarily, or even permanently).

Imagine defending a druid circle's holy grove, upon which they give you a leaf of their magical power herb, which increases the "power level" (i.e. effective spell slot) of any ranger or druid by 1 for an hour.

or

Maybe you saved a master artificer that gives you a magical pendant that grants anyone who wears it a spell slot of 6th level.

To allow DMs to make these kind of choices they should also limit any spell in power.