If you're going to create an entire universe—in six days, no less—it pays to have a set of blueprints. Enter the map, God's little crib sheet covering every corner of his vast creation, from everything that was to everything that will be to everything that might be.
The map also discloses the location of various "time holes"—botches in God's plan that have made openings in the fabric of reality. The dwarves have been given this map so that they can go and fix the holes, but instead, they use it as a means of escape after they've committed robberies. In the process, they get the map dangerously close to Evil's pointy claws.
That makes the map something of a plot device. It's kind of like the DeLorean in the Back to the Future movies, or H. G. Wells' original Time Machine. It's there to make time travel possible, and Gilliam has given it a little fairy tale panache to make it cool. But basically, it's here because there really couldn't be any movie without it.
At times, the map morphs into something a little different—a MacGuffin, a plot device designed to set a story in motion. It doesn't matter what the MacGuffin is. It only maters that the characters want it. (Alfred Hitchcock coined the term, and there's a nifty little movie about it here.)
Prominent MacGuffins in other movies include the ring in The Lord of the Rings, the title bird in The Maltese Falcon, whatever's in that briefcase in Pulp Fiction, and every single thing that Indiana Jones has gone after. Gilliam is kind of obvious about the map's purpose as a plot device since he never actually explains what it is (though the dwarves are dumb and greedy enough to go after it, regardless).