Pitcher preferences

Veteran managers,
I wonder why I would give my starters an early hook. What pitchers should I pull early? My #3/4 starters? What about the max number of innings per start. Why would I set this up lower than their fatigue inning?
Can you share some of your strategies?

Some pitchers suck, and the fewer innings they are out there the better. Especially if you have a solid bullpen. You keep poor pitchers on your team if they are super cheap. A .75 S6 starter with a 5.50 ERA and 1.65 WHIP can give you 4-5 maybe 6 innings and give you a chance to win with your pen. You can't (usually) afford 4 6M+ starters who can give you 7+ IP every time. But my pitching staffs, lately, haven't been very stellar, so I could be wrong.