Pessimistically for telemetry, but I suspect some of them are Javascript apps in a lightweight shell that have actual technical difficulties making a TCP IMAP connection.
That being said, there are security/privacy implications to that model, and I wouldn't personally use an app that works this way. That's why Mimestream was built as a traditional client.
[1] https://stackoverflow.com/questions/41674063/is-it-possible-...