When you enter a slot tournament you are given a session time for your play. The play is done on a set of machines that are programmed to hit at higher rates than normal. You do not put money in them. They are set for a time period, like 3 or 5 minutes, or a credit amount (I've seen both, not necessarily in Las Vegas.)
The chance to win big for a small investment: since slot tournaments can attract a large number of players, this also means that many buy-ins contribute to the prize pool. Considering that everyone has an equal chance of winning, there's a good chance you'll make a tidy profit on your investment.