Vincent Van Goh, Monet , Andy Warhol, Picasso. and a host of others have a built a body of sketch work. If you were privileged to peer into the pages of their sketches, they all had their own style and subject matter. in other words “ they all did their own thing.”
The sketches you should draw should be things you like drawing . and that could just about be anything. I would add , to benefit from your sketches ..you should think of them as a platform for creating ..sketches are greatly used by many artist to develop their ideas . many use thumb sketches , a series of small sketches to ascertain how they may want a drawing an image depicted in a final drawing, or laying the ground work for a painting.
so as to benefit from the sketches you draw and that means draw what ever subject matter you want , from drawing lines, circles, squares to buildings, mountains, people, cars . The list can go on and on, which is good practice sketching. I believe the key that has worked for me is it helps me to develop ideas or to prepare me for how I may want to place the various images on my paper for the final drawing or painting.