Netlify is a large proponent of JAMstack and has done a lot of work towards pushing it forward. It’s just a service that makes static site deployment simple and easy. The downside for not using Netlify is you have to add some logic to handle the stuff Netlify abstracts from you.
With a basic usecase, you might have your frontend source managed with git, via a service like Github, and your content managed through Ghost. Every time you make a change to your production Github branch (as an example), Netlify will rebuild your site, and every time you make changes in your ghost publication (i.e. when the site.changed webhook is triggered) Netlify will rebuild your site as well.
All of this happens through webhooks. Netlify manages all of the logic related to the incoming webhooks (e.g. authenticating the request and triggering a new deply), as well as keeping your site live and working (e.g. a build fails). If you were to do this by yourself, you would have to implement the logic that Netlify abstracts yourself