sometimes you can feel where it is in your voice, if you sing or hum a lot. maybe that's why the greats start humming more. they match the pitches they are hearing and just fake it if they lose their spot. i wish i could fake it. i have to use the composer's notes and put them randomly together to get back to my spot. if i had perfect pitch, i would use it a lot in performance. i would use it for transcribing music, also. (just sit/listen/write).
and, if one ever wants to be a good conductor (which many pianists become), you have to know these things. choirs tend to rely on the pianist (for some reason) to know everything (especially if the conductor is late). you have to warm up with the same exercises you heard yesterday. i go home and try to write them down and put in my book. if i had perfect pitch, i would repeat them verbatum without having to write down.
and, in the church i am now attending, they do everything (it seems) except for a few pieces, by rote. that means a lot of jazzy interludes between the pieces and sets (which i cannot do yet, but sometimes have time to practice an interlude - going throught theory of what key the last song was in, and where i have to go and what to do to get to next key. i am absolutely terrible at this and want to get better.
when i was younger, when i would hear a piece in my head (composing), i would notate it on a napkin or piece of paper with interval and rhythm shorthand of my own. at least i know intervals (sort of). i get hung up sometimes on 6ths.