I built my house 4 years ago (with liberal use of subcontractors), and one of the things I knew I'd want is ethernet everywhere. So I ran the wire, bought a big patch panel, and now every room in my house has 2-4 ports for whatever I might want.

As a bonus, if I (or the next guy) wanted to have a landline phone, it's in the same closet and it can be patched to anywhere in the house by swapping a single wire.

Given the opportunity, even after the house is complete, it's a very useful thing to do.