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When you paint, you're building an entire picture purely with color (where "color" includes value.) This forces you to really come to grips with how color works.
You could technically achieve the same results by "painting" digitally, but this is not recommended for learning - there are far too many ways to fudge things digitally, and most beginners who skip traditional painting and try to go straight to digital fall into the trap of fudging things and make very little progress...
Traditional paint forces you to think about what you're doing every step of the way, so you learn more overall. You can't just breeze through it, and there are no shortcuts, and no fancy filters to make a bad painting look slick. You have to REALLY learn if you want your paintings to look like anything. (But once you've mastered a traditional painting medium, transferring that knowledge to digital is a piece of cake.)
Anyway, how ELSE are you going to learn anything about color? You won't learn everything you need to know about color by coloring in line drawings with inks, markers, colored pencils, or photoshop. It's too easy to be casual about color when the image is already defined by line work. And you certainly won't learn much if you use mediums that give you a set of predefined colors to work with (like markers.) You need to mix your own colors to understand them completely.