I can't speak for the rest, but I try to somewhat simulate a corporate network. Firewall(s), router(s), VoIP servers, VMWare, AD type servers...etc. I have not done this in awhile but am currently building it up for some Cisco stuff I gotta learn. Takes a lot of server/PCs to get everything right, then getting commercial firewalls (not zone alarm type), routers, switches and so forth adds up. Worth it though if you use it to learn.
Plus, you can do some cool stuff for personal use.