please elaborate on comment for base:Data.Type.Equality.(==)?

[Nicolas Frisby]

[CC'ing Richard, as I'm guessing he's the author of the comment.]

I have a question regarding the comment on the type
family Data.Type.Equality.(==).

  "A poly-kinded instance [of ==] is *not* provided, as a recursive
definition for algebraic kinds is generally more useful."

Can someone elaborate on "generally more useful".

Thank you.
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