Manual mode is much like using an old film SLR from the 1960s, when they didn’t have buttons like aperture priority and other modes that do it all for you.
Being the only option, photographers were forced to learn to use their cameras in manual. In doing so, they fully learned how their cameras worked.
Once you know how to properly use your camera, it becomes much easier to spot where you’re going wrong and to fix it.
Priority modes are good for some situations. But once you know how to properly shoot on manual, you’ll find there’s no need for them and you’ll get better photos on your own.