Ok, compressing after the fact is done by software and compressing before hand is done by hardware but, taking preference or analog V digital debates out of the equation, there's no difference except that you're committed to it if you do it on the way in.
Technically there's a difference in that you can track hotter - With hardware comps you can track at a level that would ordinarily clip but the comp stops it from doing so.
I'm not sure there's any real advantage to that, though, with digital recording. It's not like noise floor is a problem.
I bet that was the thing to do in the tape days?
Unless you're dissatisfied what what your software compressor is doing, or you have a piece of hardware that you like the sound of, just keep doing what you do.
I've only recently started tracking backing vocals with a hardware compressor.
The reason for buying/trying was curiosity but the reason for sticking with it is I like the sound of it and it saves me from putting anywhere between 8 and 16 additional compressors in my session.
Bus compressing them isn't the same.