It's basically a replacement for MacPorts.
http://www.pathname.com/fhs/pub/fhs-2.3.html#THEUSRHIERARCHY
and in particular:
http://www.pathname.com/fhs/pub/fhs-2.3.html#USRLOCALLOCALHI...
Isn't root supposed to own that?
In the case of OS X laptops, they're almost uniformly used as single-user machines. This just takes the hassle of having to sudo to do it.
There are a few other reasons they specifically opted for /usr/local instead of other paths here: https://github.com/mxcl/homebrew/wiki/installation