Q: what is not useful os x mavericks on my computer

It is useful.

All of it is useful to someone!

It's impossible for me to tell you what you will find useful.

If you are looking to save disk space, the Applications and System files take up a very small amount of space, and it is inadvisable to delete stuff. In that case, you need to consider replacing the storage drive or using an external or network disk for some of your data.