Jekyll is awesome, and there are lots of other great static site generators as well. I host 7 websites on github pages using Jekyll. For a few of them where other people write content, I've set them up with prose.io for editing more like a CMS.

They're simple, fast, and secure compared to CMSs. Web developers get the cognitive benefit of separating things into templates, includes, assets, content, etc. And it integrates beautifully with version control, for rollbacks and knowing who made a certain change, which can prove difficult with CMSs. It's also easy to move between hosts if need be. I'm also excited lately because I can publish all of my static sites on ipfs.

It's not the right tool for every job, but it's great for a lot of sites.