Submodules implementation

Hi,

what is the main reason that submodules are their own repositories linked into the enclosing one and not just additional pointers in the main repository?

My impression is that submodules as pointers to existing tree objects would make a design more easier to understand and more user friendly. Especially I see no need for most of the submodule commands. Maybe "git submodule add" but the other commands are already covered by existing ones.

Or is there a tool that uses such additional pointers for submodule management?

Thanks Christoph