I think any mature producer would do well to consider learning the mixing and mastering process, its such a rich and deep field of the music making process, I find it very rewarding. Ive spent many years learning how to mix, and whats involved in mastering, and even now some of my results can be a bit hit or miss. Since I invested in the headphones though my mixes have massively improved and Im still making massive realisations and having huge epiphanies. Its not really for the absolute beginner because its such a deep topic and easy to just fuck up with one wrong plugin on the master bus.
It also makes you realise the importance of going to a pro for final mastering, and even for mixing if you havent honed your skills and ears yet.