Hello, I started to use Joplin a few weeks ago and already make heavy use of the URL field.
I find it useful for not having to include this metadata in the note itself or to prevent notes from popping up during search simply because they contain that URL without being a source from that webpage..
You could also put other type of links here, for example mailto:person@domain.com (mailto has the benefit of opening a new e-mail right away from Joplin, just use an asterix when seaching to ignore the mailto part: sourceurl:"*person@domain.com")
Once you start to feel more comfortable using Joplin and begin to extend its features with plug-ins I highly recommend JackGruber's Note overview. I use this all the time in combination with sourceurl search.
Good luck and have fun exploring!