Following on from the post on Increasing Vertical Jump, this post supports the fact that strong legs are required for a good vertical jump.
Vertical jump and sprint times correlate well with max squat strength. This means that if you have a strong max squat you are likely to have a good vertical jump and short distance sprint speed (both of which are important for volleyball).
Leg strength is a big factor in the first few steps of a sprint. As the distance increases there becomes less reliance on leg strength and other factors such as rate of force development, efficiency and technique become more important. The guy with a 200kg squat does not necessarily win a 100m sprint, but he will certainly be one of the quickest over the first 5-10 metres. The first few steps are the key in volleyball, and maximal squat strength plays a big part in this.
Likewise with a vertical jump, there is a strong correlation between max squat strength and vertical jump performance. Interestingly the correlation between max leg press strength and vertical jump is not as strong. This is because the squat is a more similar movement pattern to jumping than a leg press and hence there is more carryover. The correlation between leg extension strength and vertical jump is even weaker.
The squat plays a large role in improving movement around the court and vertical jump. There are other factors which affect performance in these tasks (see post on Increasing Vertical Jump for some factors affecting vertical jump).The key is to find the one that is the weakest link, and work to improve on that, in order to get the fastest gains.
I am willing to bet that in a large proportion of volleyball athletes the limiting factor is brute strength. Laying a good base of strength is crucial, now go and do some squats.