Why Build a Custom Kernel?
These days, it is far less common to have to build a custom kernel. In the past, the main reason for doing so was for device support. Usually, one built a custom kernel because the default kernel didn't have support for a device in her system (a sound card, for example). These days however, sound card support—as well as support for most other devices—can be dynamically loaded as a kernel module. (Rather than being built directly into the kernel, the module can be loaded into the kernel dynamically at boot time.)
However, there are still times when you might want to build a custom kernel. There are generally four possible reasons for doing this:
You've cvsupped to a newer version of FreeBSD, and you need to rebuild the ...