The reason people bring it up is that it's real for some people, even if it's not a universal requirement. Anyone who sells software has customers who have standardized on one database. Some places have senior management wanting to avoid lock-in (Oracle migrations will almost always be mentioned in this case) and may have pressure or outright requirements — and if you're a contractor working for them this will be written into their requirements. Some products have a demo/lite mode to make it as easy as possible to try – e.g. SonarQube can run in a single container (with prominent warnings that your database is not persistent) just to make it easy for someone to try it.
It's certainly not the most common and I would definitely seriously question it if you know you're going to benefit from a Postgres-specific feature (the last big Django project I started used JSONField and several other features heavily), but the world is a big place and there are many features which only some people use but it's critical for them. This discussion reminds me a lot of when people would question why you need a localization framework because they've never used one.
It's certainly not the most common and I would definitely seriously question it if you know you're going to benefit from a Postgres-specific feature (the last big Django project I started used JSONField and several other features heavily), but the world is a big place and there are many features which only some people use but it's critical for them. This discussion reminds me a lot of when people would question why you need a localization framework because they've never used one.