One motivation is to separate interface and implementation. Using interface allows one to easily observe available methods in a one place. With class, one must

One motivation is to separate interface and implementation. Using interface allows one to easily observe available methods in a one place. With class, one must use IDEs to filter out hundreds irrelevant lines just to find out class contract. If you ever used Delphi or C++, it provides much better experience by clearly separating class interface and class implementation.