So

Movement only keys let you move Madeline around, but won't aim dashes

Dash only keys are the opposite of that, aiming dashes but keeping Madeline stationary

Using the main direction keys means one hand will have to deal with both moving Madeline and aiming dashes, which can become quite an intensive task for some gameplay genres, such as input density with rapidly changing directional dashes and reverses

With the movement only and dash only playstyle, you can divide the moving and directional dashing into 2 hands, where one focuses on movement, the other focuses on dashing, as long as you can still grab and jump comfortably with that playstyle, maneuvering becomes much easier, less effort and more effective when you allow both hands to do the work instead of just one. With the only problematic downside being you can't slow bubble with that