No it's not. I work in open source and very rarely use github. git is a standard tool, github is only a polished frontend that seems to be a standard amongst web developers.
It's really not. In the last 3 places I worked at we all used Bitbucket because their pricing is more reasonable and it integrates nicely with other Atlassian products like Jira.
That makes no sense at all. It's like the "track changes" feature in Word, maybe. You know--one little feature on a much, much larger tool.