During the course of your software development career, you get to use different flavors and versions of databases, such as PostgreSQL. Newer projects and applications often use the latest versions while older projects tend to lag behind in upgrading their stack.
Consequently you need to be able to run different versions of the database in your machine, and be able to switch easily between versions. The alternatives, such as re-installing the desired version when needed or using a different machine for different projects, isn’t practical most of the time.