Why does MAXDOP = 0 result in high CXPACKET wait time?

During a parallel query execution (whether or not to go parallel is decided by the cost threshold for parallelism) MAXDOP limits the number of tasks per request. Assuming cost threshold for parallelism is a low value, there by causing even smaller queries to work in parallel. When MAXDOP is set to 0, what makes the processors that have done the work wait (resulting in high CXPACKET wait time). Can they not pick up the next work and not just wait? What makes the processor wait once it has done its job? From https://www.brentozar.com/archive/2013/08/what-is-the-cxpacket-wait-type-and-how-do-you-reduce-it/ >This isn’t really a bottleneck per se – the students could go off and do other work – but they like to complain about how they had to wait around for the slow kids. > >That complaining is CXPACKET – Class eXchange Packets. The class is turning in their packets, and complaining about the slow kids.`