It prevents people from abusing faucet. Like ethan said, someone can claim faucet, invest it, withdraw and repeat. Some people are desperate enough to sit all day milking faucets, and casino faucets are much bigger than classic faucet sites, so owners should monitor faucet activity and develop protection mechanism against such abuse.

Here's another idea - faucet just gets locked for some time (6-12 hours for example) after making investment. This way investors will not be affected and most other users too.