Cause I think it's cooler, and I'm already playing a Space Marine.
In this case, it'll be a Space Marine dealing damage comparable to an OW weapon specialist with a long-las.
Sorry if it sounds like I'm bashing you for the choice, I'm actually bashing the rifle itself, as FFG seriously dropped the ball on it. I especially love how they spent half a page building it up as the ultimate sniper rifle with a ton of built-in customizations, but when you actually parse it for rules it comes out as a slightly better scout rifle with a bog-standard scope.
Except when I'm using the sniper rifle, the idea is to be stealthy. It's not an opportunity that comes up much, which is why it won't be the only thing they can do, either. It's easy to forget but the Ultra's other boon over the Stalker (aside from Felling) is that it's incredibly silent, with a -20 to Awareness and you can only hear it at half the normal range (I forget how far that is). You need to use Stalker shells with a Bolter (admittedly, they're good) if you want to be discreet. Because people exploding all over the place will tip someone off. Acids and fires aren't really subtle either.