You want to mix the beat and make it sound as dope as possible for a few reasons
1. You only get one first impression with an artist, and speaking from my own experience, I will immediately skip on a beat if it it's not sounding right
2. Leaving it "unmixed" makes it so the sound engineer has to do a lot of work in the mix to make your beat simply sound up to par, which is not efficient. you want that shit sounding dope already, so the engineer can make it sound unbelievable when mixed with the vocals.
3. Learning how to mix your own beats helps you cultivate your own sound, which is key in not sounding like all the other producers in the world. it's all about what comes out the speakers, which is more than just composition and arrangement