gibbsman - over the past couple months, SETI has undergone either severe bandwidth restrictions (meaning that client machines had difficulty connecting directly to get new WUs) or outright outages (no one could connect).
Programs like Setiqueue allow users to "save for a rainy day" and have some extra WUs stored up in case no one can get new work from SETI. That way, the machine never goes idle. Meanwhile, the queue will do all the uploading and downloading of WUs for your machines.
It serves multiple purposes in that it can act as a server for alot of machines and is helpful for those behind restrictive firewalls (where the internal machines don't have outside internet access to SETI). It also allows some modicum of stats, where you can monitor how you're doing (especially when the SETI stats are down).
Hope this helps explain them?