because you can set your own parental level to each games.
if you go to the game settings, there's a "parental level" you can edit.
so, you can even set a different level than the default from gametdb.
But you are right, it's missing a lot of explanations. the loader can certainly do things that users don't know.
We used to have a readme, but it was very old (before 2.x), nobody maintained it.
We tried to make a Wiki with proper guide to explain how to setup and use the loader, but it took too much time to write and we never completed it.
Edit:
ahh, just an information about parental control.
there is a bug with parental level for channels installed on realNAND.
The channels are loaded before wiitdb.xml is loaded, and a default parental level is assigned to these titles.
When Wiitdb is loaded, it detects that the game already has a parental level (that you could have set manually, for example) so it doesn't overwrite it with the real value.
I have to fix this, but didn't had time to do it.
As a work around, you can set the parental level manually for each installed channels. the level will be stored in the game settings and should be loaded early. (I hope. I have to re-check the sources).