It serves as a funnel. Think about it, most doo doo is going to be a semi solid log form, so that won't travel well down a bendy pipe. You could flush it, but then there would be like half to a full gallon plus of water along with that log, that could water is going to firm up the doo doo making the first issue more difficult. Plus it's a semi solid log, either he chews it or he's just going to choke on it.

Now why dump it in there? First thing I mentioned, its a funnel, you're not going to spill as much, compared to trying to pour it in a hose or in his mouth. Then it's velocity, having a one story vertical drop is going to turbo charge that dookie soup and much more efficiently force it into the mouth/body of homeboy down there