The means to genetic enhancement. If you want to purchase character upgrades, tonics, or plasmids from Gatherer's Garden vending machines, you're going to need as much Adam as you can get your hands on. That should give you plenty of incentive to harvest or rescue Little Sisters when you come across them in Rapture.
Just as in real life, you really can't ever have enough money in Rapture. Not only will you need the standard currency for purchasing items from Circus of Values and El Ammo Bandito! vending machines, but you'll also find that money is useful for trouble-free hacking, healing at health stations, and shutting down security alarms.