One major advantage is the impact on phone battery life - partly because various services aren't repeatedly pinging over the mobile network (especially in "notspots"), WiFi tends not to burn through battery. And partly because it tends to change your behaviour to do more focused things (the phone isn't constantly trying to distract you as it often can't, so you don't then waste time and power on these distractions).

I don't think I'd abandoned a SIM card in the near future but having used my old phone without the SIM as an alternate for my new phone on a few occasions, the old phone battery went from being awful to lasting way longer than my brand new phone of comparable specs.