I second @pamtbaau on getting your hand dirty.
But for having done a lot of wordpress and Joomla development.
I can comfortably say that usually a good majority of site for small and medium client do not need such CMS, and in this context, with no database.
The benefit of these flat file base CMS such as Grav, and the more simplistic or light framework such as Hugo, jekyll; is the simplicity of use and development coupled with great performance and security.
In my honest and professional opinion, where Andy and the Grav team has done great is that Grav provides you with a system that you can actually give to a client to use easily.
For devs, you have control over plenty of settings and tools. You have a good range of possibilities.
Super easy to move and install on a server.
Very user friendly admin panel plugin. Which is useful for client. But as a dev you can actually work entirely via the editor of choice.
Different configuration for different set-up. You can have your config file for your local dev, staging and prod.
Client can manage users, content, form submission, backups… Which are major factors when handling over a site to your client. He needs to feel autonomous, even though we all know they will still call you to help
Don’t get me wrong, others I mentioned (Hugo, jekyll…) are very good as well. You have more simple/lean but still efficient ones like eleventy (11ty). But in terms of “client friendly” Grav is definitely on top of the list.
I believe that as a dev, it’s always better to know a few different systems as they all have their strength and weakness. And all projects have different need. There is no “one size fits all” in web development.