Seeing as Boswell‘s father, Lord Auchinleck, was the austere, hard working Protestant type, I‘m going with the shared denominator for Boswell and Katte of: Dads not willing to finance the Grand Tour unless sons willing to do the backpacker thing of also using at least some of the time abroad for studying and hard work. Mitchell, of course, was his own man and in control of all the cash, so in his case it might have been an honest career consideration - i.e. it would be useful to have as a background once he got back to Britain and fully qualified at the bar.