At the moment it is rarely relevant...
No standard situations require that a creature utilize their spellcasting ability when casting misty step.
... but it could be in the future
If a later feature (such as a trap or a monster's trait) requires that a creature make an ability check using their spellcasting modifier for any reason, then this will be important. Say, for example, a monster that adjusts spellcasting making a caster roll an ability check with their spellcasting ability or the monster gets to choose the targets for the spell (with some DC). It is the general rule in 5e that any spell cast has a spellcasting ability.
P.S. I use this example, because I have created homebrew monsters with traits like this (among others) in my games.
For an example of a feature that already uses this type of language, take a look at the Arcane Trickster's Spell Thief feature:
Immediately after a creature casts a spell that targets you or includes you in its area of effect, you can use your reaction to force the creature to make a saving throw with its spellcasting ability modifier.
While misty step can't target an enemy rogue, other spells that typically don't directly utilize a spellcasting ability (such as magic missile) could. In these cases, the spellcasing ability would be used to determine whether the spell is negated and stolen by the rogue.