Using a quality tablet there are things which you can do with it, which you simply CANNOT do with a mouse or a cheap tablet. People can argue about comfort, inaccuracies and other things in the endless mouse vs tablet debate but -
With a mouse you only have two parameters to input - X & Y direction.
Cheap tablets generally add pressure sensitivity, which means you can do something simple like set pressure to brush size, this way you can draw a fluid thin to thick line, you just can't do that with a mouse.
Dependant upon the quality of your cheap tablet, you'll have different quality of the detection of sensitivity.
Going to a quality pro tablet, then also adds tilt sensitivity, you can now not only draw that fluid size changing line, but also say link tilt to opacity and fluidly change that at the same time. Some pens also support rotational detection as well, meaning you can twist a brush at the same time.
IMO, this pressure / tilt combination is totally invaluable. It either allows me to do things I simply couldn't with a mouse and / or speeds up editing time by at least a factor of 10.