Why did animators trace over filmed actors?

Because tracing real movement was a shortcut to realistic movement. Rotoscoping is an animation technique in which animators trace over motion picture footage, frame by frame, to produce realistic action. Originally, live-action film images were projected onto a glass panel and traced onto paper. The animator Max Fleischer invented the technique in 1915 and used it in his Out of the Inkwell series (1918-1927).

The first star was Koko the Clown. The live-film reference for the character was performed by Max's brother Dave Fleischer, dressed in a clown costume. Frame by frame, the filmed brother became the drawn clown.

The shortcut had a price. Conceived as a shortcut to animating, the process proved time-consuming because tracing is precise and laborious. Misinterpreting the forms made the line wiggle, so tracings had to be reworked, using them as a guide where consistency and solidity were important. By 1924 Fleischer had stopped depending on the rotoscope for fluid action.

In the 1930s Fleischer returned to it to reference intricate dance movements, including the routines of the jazz performer Cab Calloway in Minnie the Moocher (1932). When Fleischer's patent expired by 1934, other producers could use it freely, and Disney used it extensively in Snow White and the Seven Dwarfs to make the human characters' motions more realistic. The film went significantly over budget because of the complexity of the animation.