There are lots of answers for a lot of circumstances. Yes, timber you buy at the store is generally planed and uniform, and if it happens to be what you want and need, then good. But as you go on into more in-depth work, you will find that the cookie-cutter sizes become less and less satisfactory. As far as various odd sizes and getting them uniform, again it depends on what you are needing. You can certainly plane a 1″ board down to half inch, but usually you are just wasting materiel. In such a case, you would cut it down close and plane it to exact size. This helps ensure it is flat and removes tool marks. Or, like you asked about, you may have several pieces of timber that you want to use on a particular project that are all wonky sizes and just work them to a uniform thickness, regardless of what that may be. At least for me, that comes up fairly often.
It comes down to not being limited to what Lowe’s offers off the shelf. Do this stuff long enough and you will have a barn or shed full of timber in non-standard size and you can pick out something and say “Aha, this will be perfect for that.”
As far as planing and straightening, Josh has a good video on that.