They are the most accessible planetary bodies in our solar system. It's counterintuitive, but they're even easier to get to than Earth's Moon, for a robotic mission.
That's right; the space race isn't dead, it just has real problems with the concept of distance. Image of Phobos courtesy of ESA / DLR / FU Berlin (G. Neukum)
Mars's tiny moons - One small step for mankind? [New Scientist]