Right. The Man isn't paying you any interest. So, if you give him more out of your paycheck than he asks for, he's getting to hold onto it until April 15th of next year, then giving it back to you without paying a dime. By not paying any more than you need, you get more money in your pocket, which you can then invest and make a return on. However, if you're irresponsible at investing or spending, it can be a drawback. Which is why most people like to get big tax returns. If they had just gotten that much more money spread out over each paycheck, they would have spent it. Instead, they get a nice big check to get something nice with, or to invest.
The smart investor knows how to use interest to make himself money. For example, when my grandpa buys something, if they offer no-interest financing, he gets it. Then he pays the minimum for as long as possible. Then, at the latest date possible before getting slammed with all that interest, he pays the rest off. He easily could have paid for it in cash, he's a fucking millionaire. But instead of being in someone else's pocket, he was able to have that much cash earning him a decent amount of interest.