This package is a handful of the mouse event functions from Html.Events, except they return the position of the mouse click relative to the element itself. So if you were to click in the middle of a 200 x 200 div, onClick would pass along Point 100 100 to its Point -> msg.
We made this package for Elm-Canvas. But it became clear that mouse events are outside of the scope of Elm-Canvas, even if many use cases of Elm-Canvas could use some mouse events
mbr has a good package that does nearly the same thing as this package. The key difference is that mbr's returns a record with the target and client positions that can be used to figure out the element-relative positon of the click. This one tries to calculate that behind the scenes and just return the position.