Utility of Proficiency Bonus in Statblock for Summoned Creatures without Proficiencies?

Question

Looking into Tasha's Cauldron of Everything, I was reviewing the stat block of the Fey summoned by the Summon Fey spell. Amongst the statistics provided is the Proficiency Bonus of the spirit, defined as "equals [the caster's] bonus."

The Summoned Fey has no Skill, Tool, or Saving Throw Proficiencies. Presumably the Proficiency Bonus value is already effectively baked into the Shortsword Attack, and the Spell Save DC for its bonus actions, as those are based on the Caster's Spell Attack and Spell Save DC.

Is there another reason why the Player or DM would need to know this Proficiency Bonus, or is it merely a leftover from using a similar Stat Block for companions that do have proficiencies, such as the Battle Smith's Steel Defender, or the Homunculus Servant Infusion?

Answer 1

Every creature has a proficiency bonus

Every creature is required to have a proficiency bonus for a wide variety of reasons. In the specific case of the Fey Spirit, none of the usual reasons apply, but there are still rare instances where it is important to know the proficiency bonus, even for creatures who don't have any traditional uses of it.

One example for the sake of illustration is the Axe of the Dwarvish Lords which says:

You gain proficiency with artisan’s tools related to blacksmithing, brewing, and stonemasonry.

Obviously, this is an exceedingly rare case, and conjured Fey Spirits can't attune to the Axe normally since it takes an hour to do so (the duration of the spell). However, it helps demonstrate that proficiency bonus is a vital piece to a creature's stat block.

It is easier for Wizards of the Coast to ensure that every creature has a well-defined proficiency bonus than not. This allows them to create features in the future that will utilize these proficiency bonuses without having to retroactively add them to any creature without a well-defined one.