An example: Many corporations will have corporate wireless hotspots. However, in order to make sure people abide by their internet policy (and don't open themselves up to all sorts of abuse) they will get users to go through their proxy server, which among other things will make sure the user gets a level of filtering mandated by the organisation. Users need to point their device to the proxy server, before the network would allow you to talk to it, or to the Internet (even after authenticating) and this can be done in several different ways.
On Android, until recently, you just couldn't do it. Newer firmware releases can, but its a half-assed attempt at best.