Instead of just placing MIDI notes to create melody or groove you can also use velocity to further add expression.

By default MIDI velocity normally controls volume but you can get creative and take your programming up a level by getting velocity to control elements such as filters, envelopes or even effects for example.

This is just one of many ways to add expression. My personal favourite is to assign velocity to filter cutoff, decay and drive so the harder a note is pressed the more aggressive and open the tone feels.