From the manual:
The default minimum framerate is 30 FPS, meaning the maximum dt is 1 / 30 (= 33ms). If the framerate drops below 30 FPS, dt will still not exceed 1/30. This has the effect of the game going in to slow motion as it drops below the minimum framerate, rather than objects stepping further every frame to keep up the same real-world speed. This helps avoid skipped collisions due to stepping a very large distance every frame.
I'd rather have my game slow down than skip frames and risk skipped collisions or other issues, so I set mine to 55 because framerate usually fluctuates a bit.