That's why the application has a limited number of features - basically just different types of paintbrushes, a complete color palette, and a layer system. The program displays a simple interface, so you can focus on your creativity and get right to work.
The program has the advantage of being very lightweight compared to other, similar tools. It takes up very little space on your hard drive, and barely consumes any memory when you run it, so you won't have any problems listening to music or playing a video while you work.
MyPaint is an excellent tool for digital artists: it's free, lightweight, and performs its purpose with perfection. If you draw with a graphic tablet, give it a try.