Quick way to save your place in the ide and come back to it easily. Rightclick a line and choose Add/Remove Marker, go off somewhere, rightclick and choose Jump to Marker.
Here, using 4.10b3 if you uncheck the box you don't get the line marker actions in the rightclick menu and the keyboard shortcuts for them don't work either. With the box checked, they're both available and working. If there's anything unexpected happening with this I can't reproduce it.
You're right, I had the keyboard shortcuts swapped. They are still working with the box unchecked, though the menu items are gone. This may be intended as the only reason I can see for disabling line markers would be to keep the rightclick menu uncluttered.