Think of this:
You are searching moddb for a very specific mod.
You do not find what you were searching for, so you start searching google (or better, let google search through the internet for you) . No results either.
What now if there was a mod for a game that was not, as most mods, complete but just a core module similar to the mods for Minecraft.
I do not talk about some weird installation stuff or incompatible versions, I just mean that there could be one major call for those files and if those files (new Content) are present, they get loaded.
So, the person wanting to create his/her own mod only has to download the content files he/she wants.