The answer to this question depends a lot on what exactly you want to do as far as recording. As
@dvs mentioned, a USB microphone has an interface (at least for recording) built in. An interface will also give you the ability to play back anything from your computer through an external device, ranging from a set of headphones to a massive audio system.
The advantages I see may not appeal to you at all.
I can record a dry guitar signal using an interface. That signal is free from any outside noise, such as lousy room acoustics, barking dogs (of which I have two), a relatively noisy HVAC system in my house and road noise from all the 4x4s in the area in need of new mufflers.
It also gives me the ability to take that dry guitar signal (essentially plugged into the DAW) and process it using the built-in modeling capabilities. In other words, I can take a complete guitar performance and "shop" all the different amplifiers and effects that are available through the various software and hardware I have at my disposal.
If you record your guitar using a microphone that sits in front of your amplifier, you are recording the entire signal chain of your guitar, any pedals, the amplifier and any internal effects along with anything else the microphone may pick up. That may or may not be important to you. There is no right answer.