lots, see the jpdl documentation what can be done on a tasknode (especially look at the attributes on the xml)
Main thing is that multiple tasks in one node are (to some extend) related e.g. behaviour when one task finishes
I mean, what use case that needs to be fulfilled with multiple tasks in a task node that can't be fulfilled with multiple task node with 1 task each ?
Lots, see the attributes on the task-node. But in some cases it is simplicity (convenience). E.g. instead of a fork, multiple tasks in each leg, an n-out-of-m join with n being 1, you can have one task-node with the signal='first' attribute.