Tons of reasons. A state still retains general jurisdiction. Consider eminent domain. I want to build a new Wal-Mart, and I want the government to help me get the property from unwilling sellers and build sellers and give me breaks on stuff to incentivize me. I can buy a council and enough legislators for it. I don't need federal involvement at all, and it would be prohibitvely expensive.
This kind of thing happens all the time. I know a town who, just this week, spent 15 million buying land, relocating the businesses already there, demoing the land to Wal-Mart's specs, etc. Wal-Mart gave them each a small donation for their time. In addition, in the last election opponents seemed to drop out for no reason. The rumor is Wal Mart bought them off to not run. They were going to get all kinds of tax and utility breaks, and had a deal in place to buy the land for 4.5 million. But one thing the city forgot to do was test the soil. When Wal-MArt did due diligence and found the soil was contaminated, they left that town holding the bag. Now the town has spent $15 million for a contaminated lot that no one will buy.