I like mixing my own paints much the same as I like mixing my own dyes. I like coming up with new colors even if it means I’m not likely to reproduce them. In watercolors its especially fun to play around with mixing and come up with browns…without using brown.
Of course the standbys that are already premixed for you are easier but it’s fun to see what unique ones you can come up with when you do a little mixing. Sometimes when I want to paint but I don’t have anything in mind, I just start making dots of full color and blending them together to see what I get.
This relates to what I do as a fiber dyer too. Sometimes I forget how quickly things can turn brown. Or what if I want a new brown or a new grey. All of this mixing helps me to make new dye colors too.