I've noticed it tends to be on the more expensive/better featured boards (the same ones that might have extra SATA, or even wireless networking), I guess it's a feature they can add for very little but can make a big different in some peoples decisions on what to buy.
People with home networks can use them to share a connection easily, they might be very handy for some workstations etc and it gives a level of redundency - very occassionally you'll find something that just doesn't like one brand of nic or nic driver (IIRC a particular Marvel chipset* had problems with WoW, another type had troubles with some UO servers** and those are two examples I know of from personal experience).
It's also as already mentioned, very handy for connecting other bits of kit to locally, without sharing it over the main network (or without running another cable).
*It was used in the WRT54GS V1 or 2, and required a firmware update to solve it (it basically stopped WoW from working after a minute, but without dropping the connection).
**I think it was a Marvel Yukon gigabit chipset, when used with a certain driver would have the same affect on some of the UO servers (took me days of fiddling to sort that out - eventually by trying other machines on my network on all the UO servers and finding out that it was only one machine on the servers located in one of the then 4 server locations).