3) You have a small patch and want to update it to the HEAD version before submitting. You worked with the source you obtained in some other way (from a source package for example). Now you have to wait 10+ minutes on a large project, downloading all the changes since 1970, just to change the newest version of a single file.
4) The project has grown into several sub-projects - but you don't control that repository.
"shallow repository
A shallow repository has an incomplete history some of whose commits have parents cauterized away (in other words, git is told to pretend that these commits do not have the parents, even though they are recorded in the commit object). This is sometimes useful when you are interested only in the recent history of a project even though the real history recorded in the upstream is much larger. A shallow repository is created by giving the —depth option to git-clone(1), and its history can be later deepened with git-fetch(1)."
There are still issues; you can't commit for instance, but you can update to HEAD to update your patch, with a depth of 1 you only get the most recent changes.
4) The project has grown into several sub-projects - but you don't control that repository.