Usually, it's used in single-sign-on. Some guys in this page have a detailed explanation.
Yes. The framed-ip can also be used in that way except accounting. I've thought about this before. It's useful when you have only one AP. Because the ip assigned by radius server only need to concern one LAN (such as 192.168.1.0/24).
However, if I have many APs, the radius server may need to assign different IPs to different LANs(such as 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24).
When I writing this, I suddenly realised that it may be a easier way to get the user-ip mappings. I may have a try by this method.
I don't know why I used to think freeradius can't assign ip pool according AP. It's my mistake.