They are useful for when you want the semantics of UTC but still a human readable time. One example is logs that people will actually read. People have a hard time doing mental math but 14:00-05:00 is just as valid as 19:00+00:00 and easier for people in Eastern Time.

It's also useful for disambiguating a local time:

2018-11-04T01:45-05:00 America/New_York 2018-11-04T01:45-04:00 America/New_York

That said, I do think they are mostly around for less noble reasons: they are way easier to serialize than actual time zone rules.