And why is it necessary to setup a uke? Is it only necessary if they were made poorly to begin with?
That is the $1,000,000 question! Different people have different answers and it’s all a matter of degree and context. For example, one of my ukes was custom-made, and it didn’t require any setup at all because the “setup” was part of the building process. I also have a few factory-made ukes that had a slightly higher action than I like, so I sanded down the saddle a bit to lower the action, but they haven’t been professionally set up. If you are happy with how your uke feels, then it is probably fine, but it’s probably a good idea to play a uke that has been professionally set up for the sake of comparison.