I've participated in a volleyball tournament, and noticed that servers (especially those doing overhand serves) bounced the ball twice before starting the serve.
Why do servers do this?
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It's just a ritual.
Some players hold the ball straight in front of them (like Marcus Böhme, Maxim Mikhailov), some bounce it, some even talk to themselves (like Christian Fromm from the German National team). It helps concentration. It helps you focus on your serve and it let's you take full advantage of the eight seconds available to perform your serve. Some beginners tend to hasten doing their serve, resulting in a lack of focus and increasing the chance of erring.
Some players don't have any ritual at all, though.