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

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.