As said, this is the point where the injection happens, usually known as a gate.
Sometimes, LEGO tries to have in a non-visible place, but the process of moulding is extremely complicated and forces specific requirements on the gate placement which mean it's sometimes impossible to hide it. The basic rule of thumb is that the gate is usually near the center of the part (in terms of quantity of plastic) so that the molten plastic reaches all ends of the element at the same time. Sometime a part even requires two gates.
In the case of your slope, it's probably annoying, but sometimes it's worse. In this glass panel for example, the gate is right in the middle. It wasn't really a problem in the beginning, since that glass panel was used in combination with a three panes window frame and the gate was hidden behind one of the bars of the frame.
Then they released Cafe Corner with full window frames and the gates were in plain sight. In the middle of windows. Now that's what I call ugly.