A good movement holder is an essential tool of every workbench.
With it a movement can be held safely, which allowes secure and precise working on it. Especially the insertion of multiple gears at the same time is much easier then.
But be careful! There are also cheap movement holders, which are complete junk. The example below shows a 10 Euro movement holder, which is that crude and imprecisely made, that it cannot hold any movement.
Furtheremore, there are also plastic holders in surplus shops. They are very useful, if you don't want to risk scratches on the sides or if you want to fixate a movement by its dial:
Costs: 20€ (Bergeon 4040)