> Why even bother with such "update"?

This is a pretty usual practice for GPUs; one generation has a new architecture and new chips in the mainstream/performance segment, the next generation uses the same chips with some minor tweaks. The lower end often stays on the same architecture for longer. All GPUs get the same marketing number be it "GT 5XX" or "RX 4xx" to have a consistent line-up. It's just marketing, really.