Several applications use the user nobody as a default. For example you probably never really want say the apache service to be overwriting files that belong to bind. Having a per-service account tends to be a very good idea.
Getting apache to run as nobody:nobody is pretty easy, just update the
Group. settings. But as I mentioned above I don't really recommend that particular user/group. It is entirely possible that you may be tempted to add a service to the system at some time in the future that also runs as nobody, and you will forget that have given write access on the filesystem to the user nobody.
As for why you needed to do that for an upload directory. You really shouldn't need to change the owner of the directory for uploads. I would only change the group, and then setup permissions to 2775. Of course if you setting up a system where many different users share the same web server getting the permissions setup for the best security can be a lot more complex