From my understanding, ipvlan l3 is needed/used when you want to do something way more complex. It essentially turns the host into a router, so you get a bunch of complications because of it - like your containers not being able to access the internet, because upstream routers don't know how to route traffic back to you.

You will never want to do this as a developer, as this feature does not target you at all. You will want this as an infrastructure/networking nerd if you would want to optimize/customize the network. Think along the lines of kubernetes, but even that uses a way more complex networking setup + it "just works", meanwhile ipvlan l3 leaves you half way there.