It's for newer GPUs that need different power connectors. If you're building new you should probably get one. Old PSUs work but you might need adapters which is annoying

New standard handles power spikes better from modern graphics cards. I got a PRIME TX ATX 3.1 for my build because I didn't want adapter cables everywhere. The new connector gives cleaner power delivery to the GPU. Your old PSU will work with adapters but why deal with that mess. If you're building fresh just get the new standard and be done with it. Future graphics cards will all use this connector anyway. Might as well get it now instead of upgrading again in a year. Saves hassle long term even if it costs a bit more upfront