People use manual mode to make their lives easier, or at least that's why I do. Very simply, if the incident light isn't changing then the exposure settings shouldn't be changing either. But if your subject keeps changing, or your background, then the camera's exposure meter will get upset and try to fix that for you by changing the exposure settings. Well that is not what you want to happen and it is tedious to have to keep dealing with it. With manual you simply set your exposure correctly at the beginning and, until the light changes, that's the end of it. Here are some examples....
You are shooting a wedding group and have found yourself some nice open shade. It is blue skies above and there are no clouds. The lighting will not change at all, for your purposes, for at least 30 minutes. You set a manual exposure that is correct for the incident light. Let's say you have f/4, 200 ISO, 1/400. That will give a 0 reading off a grey card. Now you wheel your subjects in front of you.....
- First the bride, dressed in white. Focus, compose, shoot. No messing with exposure.
- Next the groom, dressed in black. Focus, compose, shoot. No messing with exposure.
- Next the B&G together. Focus, compose, shoot. No messing with exposure.
In each of the three frames the scene has an identical exposure. The background is the same in every shot. The foreground is the same in every shot. The bride's dress and skin tones look just the same in shots 1 and 3. The groom's suit and skin looks the same in shots 2 and 3. How easy was that?
Now you can carry on and shoot anyone and everyone else with exactly the same exposure. No metering required. No fiddling with exposure compensation. Every shot the same. Every shot correct. Sure, if you end up with two or three rows of people you may stop down for some more DOF. Easy peasy. 3 clicks to the right for the aperture, 3 clicks to the left for shutter speed. Carry on. It doesn't get simpler than that.
Now change the shooting scenario. You are shooting motorsports, with the sun at your back on a bright, sunny day. Vehicles of all colours and tones are passing before you, and you are getting some nice frame filling shots. Set your manual exposure, based on a variant of "Sunny 16", and away you go. Black cars stay black, white cars white, silver cars silver. The track stays the same shade, as does the grass and the spectators in the distance. You can keep shooting for at least 30 minutes, maybe a few hours, without adjusting exposure at all. Every shot will be perfectly exposed and there is no faffing about with metering patterns or exposure compensation.
Now you move on to shoot team sports, one team in yellow, one in dark blue. Once again, set a manual exposure for the incident lights and it won't matter if the viewfinder is filled with yellow shirts or blue ones, or any combination. Your exposures will remain constant and the pitch, the surroundings and the players' skin will all be well exposed.
Now you want to shoot BIF. If you are anything like me you prefer to have your birds sunlit. I know I do. I want colours to shine and contrast to look strong. I don't want dreary, flat, drab, featureless subjects and scenes, but I digress. I like to shoot with the sun behind me. No matter where my subject is within a scene, or what the background, my subject will be lit by the sun, or I won't release the shutter. It may not be full, dazzling sun, but whether the bird flies above the treeline, or below it, I want an exposure that is correct for the lighting on my subject. I don't care if the background is a blue sky, or broken clouds, or a thick forest, or open water, and I don't want my camera to care either. The exposure I want is the one that suits the incident light, not the light reflected back off the scene as a whole. Locking in an exposure manually means that the background can do whatever it likes. My bird will stay correctly exposed.
By the way, if the light was overcast (which also doesn't change brightness much from one moment to the next) then the same principles apply. If the light doesn't change then there is no need for the exposure to either.
Yahdah, yahdah, yahdah.
Shooting indoors, ambient light levels inside are often pretty steady and even, especially once the lights are turned on indoors. Set a manual exposure for the light hitting your subjects and you are good to go. Just like the wedding shots it won't matter what people are wearing. If a brightly lit window appears in the background of the frame it will not affect your exposure. The meter needle can do whatever the hell it pleases. Your exposure will be locked to suit the lighting within the room. If you get wall lights, or table lamps in the shot they won't change your exposure at all. Your subjects will still be exposed correctly.
When light levels are constant, or changing slowly/rarely, then manual mode can save you from a ton of pain. The problems come if and when the light levels fluctuate a lot. That is typically the case when you have broken cloud cover or things moving in and out of shade. In such situations you may find autoexposure better suited to the task, or you might simply have two settings memorised, or stored, and toggle betwen them. Alternatively, you may simply chose to only shoot while the subject is sunlit, or perhaps only when shaded - that's an artistic choice. If you make that choice then you only need settings for the conditions you wish to shoot in. You just wait it out when the lighting is not right.
Of course, it is not impossible to change manual settings from one shot to the next, if that's what is needed. Once you have your exposure fixed you still have freedom to adjust your composition, either by panning around or zooming in and out. Whatever you do, so long as your exposure is correct for the light hitting the subject/scene then you have one less thing to fret over when it comes to taking the shot.
Manual exposure is not about being macho. It is about being smart. More times than not it will do you big favours.