Working in manual means that it is
you making the decisions, not some bloke in Japan who programmed the auto functions.
Every time you make an adjustment, a different image will result. By learning how these adjustments affect the results, you build up an understanding of how the different settings will produce different kinds of photograph. You can choose, for instance, a deep or narrow depth of field. You can introduce some blur - or eliminate it. You can use (or know when it is not possible to use) ambient light. You can deliberately introduce an element of over- or underexposure, produce silhouettes, expose for dramatic sky or flowing water.
In other words, if you learn to use manual settings, you may learn how to take photographs, rather than hold on to your camera while it takes snap shots for you.
I wonder which you'd prefer...
This is not to say auto functions have no place in photography. They do. But knowing
when to use auto, and when not to, and which auto function is likely to give you a better result than using manual in a given situation, is all part of the art.
Learning this stuff can take, ooooh,
days!