Access to all the pretrial litigation and the reasoning behind the judge's decisions about what stays in and what stays out and how the trial will be structured and all kinds of shit you don't see as a spectator in the courtroom. Info on how the sausage gets made - judges have a different perspective from either of the parties and invariably shift the focus around a little bit - you see that from the inside and learn why they do that, rather than just react as one of the parties.
Yeah we had sort of a "what'd the attorneys' do well and horribly" session for trials.
Also we were allowed to be part of sidebars when the issues pertained to something we worked on. That was cool -- attorneys would argue, then go sit down, then we'd discuss with the judge what he should do, and then he'd make a decision.
mag clerk here.. I spend a lot of time doing stuff any reasonably intelligent high school grad could be trained to do in a couple of weeks. It probably is more practically applicable training for a typical litigator, but I think DJ clerks spend more time doing serious research and analysis. Most of our motions end up resolved by asking ourselves "is this discovery request reasonable under the circumstances? yeah, I guess it is".
I think that's the problem with being on the side of resolving discovery disputes. For the most part, on the litigator side, I tend to see discovery liberally granted unless the requests are clearly overreaching or there is something else at play, e.g., privilege or privacy problems. Every once in a blue moon there will be some interesting issue to brief, but the vast majority of the time it's just one side overreaching or the other side stonewalling because the clients don't want to give shit up or the attorneys just want to play hardball.