Starting & Maximum Hit Points Equal 5 + 10*Spell Level

When you first cast the spell upon learning 2nd level spells, the steed is summoned with 25 hit points (base of 5, plus an additional 10 per spell level for the spell).

Remember that the steed is not a player character and is not subject to the same rules for hit points that players are. At best, it should be considered a monster and the book is specifically telling you how to scale its hit point growth.

The primary purpose of spelling out its Constitution modifier as well as number of hit die is that there are a multitude of abilities like the Healer feat:

That creature can expend one of its Hit Point Dice, and you then roll that die.

which directly interact with those elements of the stat block and thus they need to be defined.

Starting & Maximum Hit Points Equal 5 + 10*Spell Level

When you first cast the spell upon learning 2nd level spells, the steed is summoned with 25 hit points (base of 5, plus an additional 10 per spell level for the spell).

Remember that the steed is not a player character and is not subject to the same rules for hit points that players are. At best, it should be considered a monster and the book is specifically telling you how to scale its hit point growth.

The primary purpose of spelling out its Constitution modifier as well as number of hit die is that the steed will be around long enough to benefit from resting and thus there's a need to define those elements.

Notably, this kind of language regarding hit dice is missing from other spells which summon an ally like Summon Fey, Giant Insect, Summon Aberration/Construct/Elemental, etc. Those spells do follow the base formula, however, they all have durations which are short and most likely preclude any kind of healing that would come from a Short Rest. However, there is an edge case where someone tries to heal them using the Healer Feat:

That creature can expend one of its Hit Point Dice, and you then roll that die.

Even still, this being an edge case is likely to be readily resolved by virtue of the creature lacking any stated hit die in their stat block and thus ineligible to benefit from the feat.