When browsing a repo that has submodules with relative urls they do not get linked like a submodule with an absolute url does.
I have setup a demo here: https://github.com/apmorton/submoduledemo
Note that all three submodules point to the same repo, but one uses an absolute url and the others use relative urls.
The aboslute url submodule becomes a clickable link that takes you to the submodule repo, the others do not.
Using relative submodule urls is particularly useful when people on your team use different protocols for git (some git over ssh, some https), as the submodules will be cloned with whatever protocol the parent repo is cloned with.