A better question, is why do you need it built in?
Use of modules is much easier, and a lot of benefit in the general case - no wireless, no need to have it. It also allows the module to be replaced if necessary without having to rebuild everything.
At most, you might need it added to an initrd (along with any needed loading rules), but otherwise having it as a module works just as well as builtin.
The only time I can think of is for some embedded use where there is no initrd...