GitHub is where developers go to find the software they use on a daily basis, and find the libraries they use to build even more software, which they usually put right back on GitHub.
I use the public GitHub many times a month. More importantly, every single job I've had for nearly the past decade has stored its code in GitHub. Not publicly accessible, and rarely at a private github.com account; every place I work uses a privately hosted GitHub Enterprise server. I wouldn't even consider using anything else, even though GitLab (and to a lesser extent, BitBucket) offer exactly the same functionality.
GitHub was groundbreaking when it arrived. Some people are having an emotional reaction and defecting to GitLab this month (for reasons that escape me), but GitHub has all the inertia. More importantly, GitHub has a vast community. It's just where code lives now.
I'm curious as to what sort of development you do or where you work that you're not a regular GitHub user. It's inescapable in every software company I've worked at since its inception, and as a consultant, that's a huge number of companies in nearly every industry. The only place I've worked in the past decade that didn't use GitHub was an all-Windows shop that used some horrible old proprietary revision control system whose name escapes me. Do you use git but not use GitHub (public or private or enterprise)?