Many players tend to stick with the same move after a tie, so there's a decent chance my opponent will play rock again. Even if they try to anticipate my move and choose paper (thinking I'll choose rock again), I can at worst tie. If they try to be clever and choose scissors to beat my anticipated paper, they would win - but this is less likely in early rounds. Paper gives me a good chance to win if they repeat rock, while limiting my potential loss.