Although with a flat pick, you can play faster than you can usually with your fingers. At least with a thumb pick, I can revert to a flat pick style by pinching my thumb pick and using it like a flat pick. Because it's a thumb pick, I don't have to use up two fingers all the time. It leaves the rest of my fingers free to play anything I want. That's a really neat feature of having a thumb pick.
With finger-style guitar, you have anywhere from 4 to all 5 fingers that you can play with. You can play one string individually or you can play strings together at the same time. You have a lot of options that you can't necessarily do with a flat pick. That's some very general basics on playing finger-style guitar.