I use M when the lighting is confusing. Example1. Indoor basketball. The lighting on the court is consistent. But it the scoreboard or the illuminated sign on the scorekeepers table is in the frame, the meter will adjust for the light and underexpose the players. So I determine the correct exposure, then set it in M, then shoot. Example2. Outdoor night football/soccer. The lighting is somewhat consistent, going down by 1 stop at the end zone. The issue is if you point the camera in the direction of the end zone, the background is BLACK. This confuses the meter the other way from indoor basketball. The meter then increases the exposure, for the BLACK background, and over exposes the players. Example3. Event where some are in BLACK and others in WHITE. The camera will increase exposure on the black clothes and decrease exposure on the white clothes, trying to get the metered scene to middle gray. Shot with musicians in black, I have to decrease the exposure to keep their cloths black, and not over expose the faces. Shot with musicians in white shirts, I have to increase the exposure to keep their shirts white, and not under expose the faces. The lighting is the SAME for both shots, just the color of their clothes. Similarly you will get a lot of light reflection off the brass instruments, and very little/none off the string instruments. The trick here is to learn where the meter works properly and where it does not, and why it does not, then adjust with exposure compensation or go full manual.