That’s a really good question. When I first started recording, my line of thinking was: “okay, I need to find a way to plug a microphone into my computer.” So I bought an SM57 and basically used adapters to get it to plug into the aux-in on my laptop. Horrible, horrible idea, haha.
I’m sure other people can explain this better than me, but basically… a microphone outputs a very, very weak signal. The preamp is there in order to amplify the signal so that you can hear it. A great preamp will give you a lot of volume without a lot of floor noise.
Then there’s the issue of converting between digital and analogue sound. Digital is the stuff your computer knows how to deal with, and analogue is more like the signal that comes out of your microphone or guitar. So the converters are how you end up with usable digital sound out of a microphone.
You can think of an “audio interface” as a handy combo of these things. It’ll also have somewhere to plug in your headphones, probably some outputs to use with studio monitors. It’s like an all-in-one box to give you an interface between your recording equipment and your computer.
As far as your question about the Blue Yeti–I believe you can use that instead, if you want. You could also use a webcam mic, a headset mic, the built in mic on your laptop, etc. I think that you’ll probably be disappointed with the quality, though. Maybe listen to some demos on YouTube and see if you like the results. I believe that people use USB mics more for things like budget podcasting or streaming or that sort of thing. If you are wanting to layer acoustic instruments I suspect it wouldn’t be very good for that.