And yet, nobody(?) admits to either using it or knowing what it could be 
used for. How very curious :-/

trio (which IMHO is a somewhat high profile uses it):

And so if I am not mistaken any project that depends on trio will automatically need to have sortedcontainers as its dependency? I am not saying this alone will explain why sortedcontainers have large number of downloads, but with one or two projects like this it will push the number up maybe?