1 Paris: So to fix the hyperdrive we need to move a three ton engine block?
2 Paris: How about we just turn off the artificial gravity for a bit?
3 Spanners: Do you remember what happened the last time we did that?
4 Serron: Hey, it was a perfectly fine jelly wrestling pit up until that moment!

In reality, generating some sort of gravity effect on board a spacecraft is pretty tricky. But it seems in fiction that it's always trivial, and that the real problems only start to emerge if you need to turn it off for some reason.