[racket] Why are some Racket functions such a 'member' designed as partially boolean?

Why are some Racket functions such a 'member' not designed as booleans?
I would expect 'member' to be a boolean.
I am inclined to write my own version of 'member' that is boolean.
Before I do, I am wondering if the bright person who designed the Racket
function: 'member', did so for some very good reason that is not obvious to
me.
I notice that there are a number of other Racket functions like this, where
they return #t or the value of the argument passed to the function, not #t
or #f.
I am just looking for the concept so that I might be convinced that I
should use the racket function as it is, rather than writing a boolean
version.
Thanks
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