localhost gives you a different security context in your browser than using a full domain name. Typically to use a full domain name locally you'd either need to mess with /etc/hosts (which can bite you later) or mess with DNS. lcl.host just makes things work out of the box.
Let me add a bit of clarification here, since you specifically asked about https://localhost
Using http with localhost is a mostly security context, but it has some quirks[0]. Using https://localhost will give you the full security context but then you're typically stuck either dealing with certs manually or using a proxy, like caddy. lcl.host should simplify your setup.
We're big fans of caddy by the way and even sponsor the project. Matt's doing amazing work over there.
