In the past, all animation was produced using 2d hand-drawn techniques. However, with the advent of computer animation, 3d illustration has become increasingly popular.
Despite this shift, many animators still prefer 2d hand-drawn techniques for their unique aesthetic qualities. 2d hand-drawn animation often has a more organic look and feel, whereas 3d animation can appear more clinical and precise.
Ultimately, the decision of which type of illustration to use depends on the creative vision of the animator.