Why is it common in the 2D game frameworks for the clear function to have the option of clearing the entire screen with a color? Why is this option available to us? Are there any practical applications for clearing the screen with a specific color aside from the default black one?
Clearing the color buffer can be optional. For example, if you know that you're going to overwrite every pixel each frame, then you might decide to not clear at all. On the other hand, some drivers might use it as a hint that the previous frame's contents are no longer needed and can be discarded, and optimise around that.
If you're going to clear the color buffer then you need to clear it to some value, and in a 2D game it might make sense to clear it to whatever background color is appropriate for the current scene. For a space scene you might clear to black, for other scenes maybe blue for the sky, or whatever.
Another reason for clearing is if you implement Quake style "noclip" as a development tool or cheat. In that case, when the player moves outside of the world it would give a hall of mirrors effect, so clearing avoids that, and you might pick a color that everything is still visible against.
Yet another reason might be as a debugging tool. If you want to ensure that you draw every pixel each frame, then clearing to something like bright magenta, then checking for bright magenta areas on screen, could be one way of doing it.