It all depends on what you have in mind to do with it.
If you have an interface, you may not need a mixer. A mixer would be of use if you had limited channels on the interface and you wanted to combine a number of inputs to cope with that limitation. If the interface is equipped with the number you are likely to use, then don't worry about mixer.
For mixing, depending on the mixer, you may be able to use it to control the DAW on your computer. However, it is probably easier to mix within the box.
Or you can take the outputs of your interface, feed them into the mixer, do mixing here, then go back into the DAW. But that's a lot of fiddling around for not much benefit.
Generally speaking, you should try to avoid unnecessary complications, because each introduced complication is another source of potential problems. If you want to focus on music, then minimize the hazards.