You lock the courses to prevent the software from displacing them during an automatic placement. During a manual modification, a confirmation is requested but nothing is stopping you from displacing a locked course. If you want to prevent the modification of the courses by other users, you must modify their access rights (Rights for the access to courses).
To avoid de-positioning a course by mistake during a manipulation, you can lock the non de-positionable courses. These courses can still be de-positioned in the grid.