I'm one... I was introduced to JB by someone else who paid out of their own pocket 18 years ago. I thought he was crazy... then a couple years later I tried it in earnest and it was ... life (well career) changing.

Out of curiosity: do you know why they pay for it themselves instead of asking their company to do so?

Because not all "developers" are developers in their day jobs. I'm personally a sysadmin most of the time, and I do my own R&D (mostly academic, but novel).

Some tools I like are not free, and I buy licenses by myself.

Cause many developers have personal projects they cant use company resources on