Some of the more advanced 'mixerfaces' have routing buttons that let you flip a channel from being fed an analogue input to receiving a digital input from your computer.Taking this latter approach to its extreme, many mixer manufacturers have started to incorporate multi-channel interfaces into their consoles, and there are many reasons why you might want to invest in a 'mixerface' rather than buying the two separately. Most mixers, for example, incorporate channel strips, headphone outputs and various routing facilities, which means that with a desk and a multi-channel audio interface, you're just a couple of microphones and a pair of monitors away from being able to do some serious multitrack recording. Broadly speaking, there are two approaches: buy separate boxes for the various different facilities you need - an audio interface, a headphone amp, a monitor controller, and so on - or get a few units that each cover a lot of bases. Before you've even decided what equipment you need, you've got to decide on how you plan to work.
Setting up a studio can be a complicated business. The prospect of combining the functionality of a mixer and an audio interface is an attractive one, for a number of reasons.