Why should be included?: It's a crucial postgresql extension to work with geographic data
Is it open source? Yes
I am developing with Django. For some projects postgis is used for geographic data. without this package the website will not work. We use it to calculating distances between a project (that is how it is called on the website) and the user to only show project within a certain radius and to show the user how far away the project is. This could be possible in other ways but that will make the query a lot slower since this is done in a request cycle.