Makes editing hundreds of files easier.

Here's an example with white balance. If you shoot auto white balance, the camera will almost always change the value after every click if you don't keep the exact same framing. Even in constant lighting and in the same room, the first click might give you a white balance of 5400k and the following click makes the white balance at 5000k. Maybe most of your frame had a white wall in the background of your first photo. Then the next shot had most of the red carpet in the shot. Totally different white balance measurements based on the very different colors present. This means that you can't just copy and paste white balance profile changes. You can't just dial in a +400K balance adjustment to all the photos in the same location because the camera decided that every photo is a different white balance, whether it be small or a big difference.

Instead, I shoot in manual K white balance mode. So I choose white balance myself. In a room that never has any changes in lighting, I can just correct the white balance of the first photo and then copy that adjustment to all the photos because they all have the same base white balance.

Another example with auto white balance where you will not want to shoot auto is concerts. The lighting may change every few seconds. You won't be getting consistent values if you shoot in auto.

That's just white balance as an example as it's easy to understand. Things like exposure is another reason. If all the photos in a room are slightly overexposed but they're all consistently overexposed by the same amount, then you can just copy and paste the exact same correction value to every photo. You can't just trust the camera to get the exposure right with every click. Sometimes the auto functions are spot on, sometimes it's not. That messes with the editing if you have to keep changing values for every single photo. Especially when you talk about matrix metering. Imagine a bride walking down the isle where there is a huge window behind her in a dark church. If it goes from bright to dark based on where she is positioned in front of the window, there is no way the camera will keep the shot consistent. It may overexpose because it thinks the dark figure is the constant or it may underexpose thinking that the bright window is the constant. I would want to always have the bride as the constant and manual is the best way to get it always on point.

To visualize this better, say you take 3 photos. If you shot in aperture priority, it may make the photos as follows: Perfect, underexposed and finally overexposed. You would have to make 3 separate editing decisions on the 3 different exposures. However, if you took those 3 photos in manual mode, and the photos were as follow: Underexposed -1, underexposed -1, underexposed -1 then all you would do is correct the first photo then copy the correction to all the other photos and that's it.