The impression I've always had is that it was a combination of an out-of-the-box secondment for the fast-track graduate training scheme, "service guarantees citizenship", and gap year "yeah, I like went to Gaul, and it was full of like barbarians and stuff, and it was so cool but you know if you haven't like been there you just don't like know, you know?" one-upmanship.

So the legate is the professional general who decides what the soldiers should do, and the centurions are the professional soldiers who make sure they do it, and the tribunes are the work experience placements who hang around getting in the way and being given non-jobs to keep them busy, apart from the occasional one who actually is paying attention and might end up as a legate - but of course the other Nathan Barleii don't really talk to him.