I also ment jamming in general. When you're playing with other people you learn how to bounce ideas off of eachother, to communicate without words, and you're forced to tow your weight- if you screw up, it shows. If you let your mistake throw you off, you leave a much more noticable "space" in the music.

It also forces you to look at what you're doing in the context of a band- I can play this crazy fill, but will it work with what we're doing? Ok, maybe it will, but how do I make it fit?

There's a lot you can only learn by playing with people and a lot you can only learn by playing in front of an audience.

Not that I wouldn't have preferred to have learned those Zeppelin tunes before I went on stage, but I was a last minute addition anyway- I had bought a ticket before they asked If I'd play bass for it, and the guy who organized the show still owes me $15.