Thanks! It helps the money get to them faster than if you just upvoted their newest post rather than it giving them a bigger payout per se.
Immediately voting on someone's new post still gives them the highest upvote curation-wise, but you can only upvote a post once. By going back and upvoting comments you have the chance to upvote them for multiple posts and instead of just giving them a 2 cent upvote, you're giving 2 cents on every comment you upvote as well. ( At least until you run out of voting bandwidth )

You can also see the value of your upcoming expected payouts and curation rewards on Steemnow.