Easing Functions have very important role in UIs, they give feeling to the inteface elements and make your application like a live enitity.

Easing Functions have developed first for Flash in 1990s by Robert Penner, then they become industry standard

Here I gathered some helpful links for developing easing functions

http://robertpenner.com/easing/

https://easings.net/en

https://cubic-bezier.com/#.17,.67,.85,-0.03