This is a little gadget I got for my b-day recently.  What it does is provide a place to plug in regular "line level" audio into and out of a computer's USB port, acting as basically another sound card. 

Why would anybody want that?  Well, the laptop that I want to be able to use for a more portable sort of studio only has a microphone input, not line in.  The problem with that is usually computer microphone inputs don't work so well for recording because they are usually optimized for use only with things like those little headset mics and voice.  They aren't the best for recording music. 

I wanted this particular type, since I have some older home recording gear like a 4 track analog tape recorder and a small mixer that can use RCA/phono type connections like the jacks you can see on this particular unit.

Would the average musician wanting to do a bit of home recording need one of these gadgets?  Maybe, maybe not.  It depends on what sort of gear you might already have, your computer, and what you want to be able to do.  If you get one of the newer mics mentioned in another thread in this section, that come with USB connectivity, then most likely not.