Being able to remotely open my editor (either from a terminal, or with X11 forwarding) with all the files I have open, exactly where I left off is incredibly useful.
A few years ago, I started using one daemon per project with server-use-tcp, each with a slightly different theme so I can easily tell them apart.
(posted from emacs-w3m)
Other big benefit is starting emacsclient is basically instant, so opening a file from a webbrowser or whatever is super fast (some folks have 30 second or longer emacs startup times).
Only real downside is it really sucks bringing back your state when emacs crashes and you have hundreds of buffers open for multiple projects across 20-30 emacs clients.
I have see this many times and never understood how startup time could be an issue. I startup Spacemacs at the beginning of the day, and close it when I leave. 10 second of startup time is nothing.
Then again it may be the case that part of the cycle is seeing if the package management system of your custom distro (be it spacemacs, doom, or whatever) picks up and configures the new packages correctly.