The answer to that depends on which game type you are having. In a demo game, it doesn't matter what the RTP is. In a real money game, however, it is crucial. Developers come out with game RTPs after simulation and several testing. Usually, the testing takes place in two phases, and independent testing organizations carry it out.
The first phase is usually about a thousand spins, while the second is three or four times the first one. While it is almost impossible for you to reach that number of spins, the RTP gives you a theoretical value for your earnings in the slot.