Ubuntu ships with GIMP as preinstalled graphics program. GIMP is a really impressive tool, once you figured out how it works. But there are a lot of people out there, who only need a fraction of the things GIMP can do. There are a lot of simple image editing tasks which can be done with simple programs:

John needs some help to do certain things on Ubuntu. He takes a screenshot of a program and tries to mark his problem with a hand painted error and some text.

Mark has been on a conference trip. He want's to remember the people he met, so he pulls a group picture from his digicam and marks everyone with a name tag.

Under MS Windows MS Paint fills this gap. Don't lough, I know a lot of people who use this program quite often. It does exactly what those people want to do. Nothing more, nothing less. For linux there are some mspaint alternatives [1][2]. But they could need some makeover (in the case of gpaint) or a targeted at a different users (tuxpaint is a program for kids).

[1] http://www.gnu.org/software/gpaint/
[2] http://www.tuxpaint.org/