AKICOLJ : what DID military tribunes actually do?

[bunn]

Although the tribune Servius Placidus keeps wittering on in my head about how important he is really, and how it is daft to have a book that is mostly about a mere Centurion, when it could be all about HIM, I am a bit woolly about what his actual job entails.  

I'm guessing he's a Tribunus laticlavius - ie, number 1 tribune destined for a political future, given the period (around 130AD). And I know that legionary staff do a lot of detailed accounts and logistics stuff.... Is he a sort of legionary accountant or management trainee?

Any thoughts?