You mean a HUD, like PokerTracker or Hold'em Manager?
If you're playing one table at a time and you are paying attention there's not much advantage. You look at the stats to confirm things (like yes he did try to steal 4/5 times, or yeah he almost always makes a continuation bet) but if you're paying attention it's just confirming what you already know.
In S&G's I pretty much only use the HUD to see the cards that people mucked at showdown. You can look in the hand history to see them, but having them pop up automatically is nice. Each stage of the tournament is different, so stats can be misleading as people change gears depending on their stack-to-blinds ratio.
Now, if you're playing six, 9-player tables, that's up to 48 opponents to keep track of. So the stats come in really handy.
Also, let's say someone who you've played with before comes to the table. You might not remember him from the thousands of other people you've played before, but now you have stats on all the
hands you two have played together. So you can start snapping off his bluffs and stuff immediately, before you relearn how he plays.
I got PokerTracker 2 back when I was playing $10NL, and it paid for itself in a few weeks.