Compare this to the benchpress, where the flat surface you are laying on provides most of the bodys support, and you can see the obvious benifits.
When performing one rep, the average human is required to lift roughly 60% of thier bodyweight. for those of us who are top heavy, its more, bottom heavy its less. Some basic physics allows us to manipulate this number. raise your feet, and it becomes harder. raise your hands and its now easier. thus a wide range of resistance can be provided.
Then you can get into the discussion of tempo. Fast or slow reps? how many per set, and how much rest between? maybe a variation of how hard you need to work? Do one set with high reps very slow, then the next with feet elevated, low reps, but done extreamaly fast?
There are near infinit combo's you can use when doing just the simple pushup, let alone its many many varients.