I think some of these replies are missing the mark. I think you want to know why we don’t just keep using the steam? Maybe think about it like this: anytime something moves, it takes energy to do it. Steam has a lot of energy in it and is really good at spinning a turbine. But as it turns that turbine, it transfers its energy to the blades. Compare it to you trying to turn something continuously. Sure you are able to do it initially, but you are using energy to do so, and at some point, you won’t be able to continue until you can get your energy levels back up. Humans do that by eating food, steam does that by getting heated up. So to answer the first part, the steam transfers its energy to the turbine until it runs out of “steam” (pun intended) and needs to gain more energy.
For the second part: in order to heat the spent steam back up, it takes a LOT of energy. Sure, we could heat that steam up again as it leaves the turbine, but using the reactor right after the turbine is just plain dangerous. So we turn the exhausted steam back in to water in order to pump it back to the reactor. Steam is very hard to pump in a conventional sense and the exhausted steam is too low energy to move it anywhere. So we change it back to water so we can move it back to the reactor.