The first reason is that you want he founders to be focused on the company, and not struggling to make ends meet. That way they can make long term decisions.
Secondly, every round the founders lose more and more control. They should be compensated for this. Yes, they have economic dilution from the investment as it is now, but some of the shares the investors buy should be founders shares. This results in less dilution for the company, and is a repayment of the sweat equity they founders have created.
One of the big problems with venture capital these days (and the reason I won't take it anymore) is that it disrespects the founders by demanding liquidation preferences for the VCs. As if the VCs who merely put up other people's money should have rights superior to the people who literally put part of their life into the company. That's not right. All shares should be equal, and if not, the founders should at least be respected. (For those firing up your keyboards to explain to me why LPs are necessary, two points: 1) I've heard a bunch of arguments for this already. 2) they've all come from VCs and are self serving. You can't really make an argument that it's better for the company or fair for the founders.)
Thirdly, too often I've seen VCs force founders to revest shares. But the value of those shares was already earned. The founder incentive to do well comes from increasing the value of the shares... and revesting the shares denies the founders crucial voting rights that they have earned already. (And makes it that much easier for the VCs to force a founder out, which is usually a bad idea in my experience.)