Some people use only a mouse to create digital illustrations, but if you personally work on illustrations for a long time, I think a pen tablet is essential.
In digital drawing, a pen tablet allows you to draw as smoothly as if you were drawing with a pencil on paper. When creating digitally from start to finish, a pen tablet is more efficient than a mouse because it speeds up the drawing process.
Some people who draw analog illustrations also create line drawings, scan them into their computers, and then apply the colors digitally.
So do you need a graphics tablet?