This is due to the desire for centralisation and (probably) security through having a separate process applying settings and changes to disk, rather than having each program do it itself. So when you make a change, for instance even to the tab you are in in the System Monitor, it will contact the running process (
gnome-settings-daemon) and ask it to write the changes to disk, it will probably also read the changes to disk via this process.
This might seem strange to you, however from a programmer's perspective (of especially modular programming) this is very good for security, as there is just one authenticated program running to make changes to disk, rather than having each and every program doing this separately. It also means that if there is a change in this process (of how the data is read to and from the disk, and perhaps even the format) that then the changes only really have be made in one place, the
gnome-settings-daemon, rather than in every single application. Again, from a programmer's perspective this is very good practice.
Why they have multiple ones running, perhaps multiple ones running for different users (for instance your user account and gdm may have different ones), it's the same thing, just under different permissions and doing it for different user space. It may also be because of something that they (the GNOME developers) just decided to do, and in terms of that, you can either take a look at the source code, or the better option, you can just ask them. But they are likely just to bog you off (depending on complexity and how you put it to them I guess).