The reason would be compatibility with the FIDE rule that determines
when two positions are considered equal.
Positions as in (a) and (b) are considered the same, if the same player
has the move, pieces of the same kind and colour occupy the same
squares, and the possible moves of all the pieces of both players are the
same.
My old 1997 paper copy of the rules adds a clause regarding castling
rights. The current online version on the FIDE website excludes this
phrase. The current version does throw in a
When a king or a rook is forced to move, it will lose its castling rights,
if any, only after it is moved.
This phrase seems out of place because it doesn't say that those castling
rights are part of the equality test in case castling is not possible. It
suggests it, but it doesn't state it. ("right" is not the same as "possibility").
Therefore, in my example, I say the castling move is not possible
anymore and thus the positions with and without the flag are equal
by FIDE's current definition.
It would be nice if FEN equality is equivalent with FIDE equality. (One way
of course is for FIDE to make its definition more precise).