Some classes (for example the standard library’s socket class) define their own send method which has nothing to do with Object#send. So if you want to work with objects of any class, you need to use __send__ to be on the safe side.
Now that leaves the question, why there is send and not just __send__. If there were only __send__ the name send could be used by other classes without any confusion. The reason for that is that send existed first and only later it was realized that the name send might also usefully be used in other contexts, so __send__ was added (that’s the same thing that happened with id and object_id by the way).