So you can plug your laptop into it at the colo when you set it up so you can verify that the server is working right and can get to the internet while you are still there. And if it isn't working right, it'll be a lot easier to keep everything plugged into the switch, rather than swapping between your laptop and the colo's network every time you try something to fix it.

But yes, if you think you might want to add other machines later, preemptively putting a switch in there will make it tons easier.