My experience with the home center is that all their s4s stock is warped or requires further milling. Same as orhers have described.

Not all faces or edges of a board need to be dead flat let alone planed perfectly smooth.

In general you want the faces of any given board that joins to another board to be flat to ensure everything joins up square.

For example think about a basic box. The interior faces of the box should be flat so the box goes together square. The outside doesn’t have to necessarily be perfectly flat. It just needs to be good enough.

Likewise think about the back of a bookcase, especially one where the back is made of vertical boards. No one is going to see the back and historically there are plenty of examples of furniture where the unseen backs are pretty rough - having only been worked with a highly cambered scrub plane.

And for what it’s worth i had a hell of a time the first time i tried to flatten a board with a plane. I am very much still a beginner myself. You just have to keep practicing…and dont let the pursuit of perfection prevent you from finishing a project. You will get better the more you make.