Reasons to switch? Most of the possible ones are listed under Features and in the FAQ on their website, if any of them are compelling enough. It also lists easy deployment, but that is arguable at this point. Running it properly on a server will most likely involve running it under a supervisor such as runit or daemontools under its own user. It does have some init scripts included in the git repo however. Also no packages for your favourite Linux dist yet.

I would pick Caddy over Nginx when in need of a capable webserver for playing around, while being too busy/lazy to set up a proper (https, http/1.1, http/2) Nginx instance.

Only if you wanted to use the automated Let's Encrypt certificate feature in Caddy.

That sounds like a very useful feature for new installations. However, I already have automatic renewal set up with nginx.