Short answer: The same reason you turn the ignition key to shut off your car.
Long answer: Back in the early days, the taskbar didn't have a Start button. (In a future history column, you'll learn that back in the early days, the taskbar wasn't called the taskbar.)
Instead of the Start button, there were three buttons in the lower left corner. One was the "System" button (icon: the Windows flag), one was the "Find" button (icon: an eyeball), and the third was the "Help" button (icon: a question mark). "Find" and "Help" are self-explanatory. The "System" button gave you this menu:
|Arrange Desktop Icons|
("Arrange Windows" gave you options like "Cascade", "Tile Horizontally", that sort of thing.)
Of course, over time, the "Find" and "Help" buttons eventually joined the "System" button menu and the System button menu itself gradually turned into the Windows 95 Start menu.
But one thing kept getting kicked up by usability tests: People booted up the computer and just sat there, unsure what to do next.
That's when we decided to label the System button "Start".
It says, "You dummy. Click here." And it sent our usability numbers through the roof, because all of a sudden, people knew what to click when they wanted to do something.
So why is "Shut down" on the Start menu?
When we asked people to shut down their computers, they clicked the Start button.
Because, after all, when you want to shut down, you have to start somewhere.
(Besides, if we also had a "Shut down" button next to the Start button, everybody would be demanding that we get rid of it to save valuable screen real estate.)