I think the idea is that hopefully, the guy giving you 5 or 6 innings wouldn't have to face the top of the order a 3rd time. These days, the plan for a starter is to get you somewhere between 5 and 7 innings. Think of it as 15 to 21 outs. Research has shown that both 1) the first inning is the highest scoring inning and, 2) starters really start to struggle the third time through the order The top three hitters in your opponant's lineup represent outs number 1, 2, 3 and 10, 11, 12, and 19, 20, 21 and 28, 29, 30. So a good reliever faces the top of the order in the first, which in theory should reduce the problem of the highest scoring inning, and... By letting a reliever get outs 1, 2, and 3, you push back the time your "starter" faces the top of the lineup for the third time to outs number 28-30.