Answer: because it is there! Cats like to push small objects around. It is part of their nature. They are probably, initially, checking to see if it is alive! And the fact that the object is on a desk is neither here not there. A cat will push it off because of a lack of space on a desk. In addition a cat might deliberately push it over the edge to see what happens.
Cats don't have the same culture of possession of things as humans. They do not have the concept of ownership and they have no use for possessions.
If the object falls and breaks, so what? Cats do occasionally knock things over to get attention. This would seem to be a learned process. At first he did it and you came running. He did it again and you came running. By this time you have been trained. Nice for the cat as he gets attention that he may be lacking.
This is not malicious behavior by the cat because for the cat the object has no value and he has no concept of the value of your time in dealing with it either.