Certain outboard compressors, to me, just have a sound or a vibe to them. There are just so many subltle non-linearities going on that are hard to replicate, I would imagine, using nothing but mathematical algorythms. And as for tracking with it .. I think it kinda' gives you a certain amount of confidence (to track with compression) and monitor it that way as you play. You hear things a little better. Over time, you can even kinda' start working the compressor a little.
You have to have a lot of confidence in the sound the comp is giving you, though, and to be able to know whether or not it's too much or not enough, etc. so you really have to put on the mixer hat early and listen and adjust as you track. You can still be conservative and ride the fader a little so you never wind up slamming it too hard. You could even do a separate dry track simultaneously so that you've got a safety on hand if you need it later.
Sometimes, it's good to commit to something. It gives you some confidence and direction ... and you know it's one last decision you're going to have to make down the road. This is how it's tracking and this is what's going to be on the finished product so I'd better like it.