I also have 12 GB. There are usage patterns where additional RAM wull be useful or even necessary on a phone. When you have more RAM, the phone can sleep tasks and leave background apps alone without having to discard their contents from RAM. This means fewer cold startups. Also, more contents can be cached, which means faster app startups. Both of these techniques also reduce CPU usage and improve battery life. You can also achieve more tabs in your browsers and more and bigger apps running at the same time. More RAM also means fewer situations where swapping is done or needed, so additional CPU and disk cycles are saved and battery usage is reduced. Some apps will actually require more RAM or spin more when memory is scarce. Examples can be advanced content creation apps in audio, video, or picture/photography. Also, some games, especially in high settings.

Are these additional GBs necessary? No. And most people would not notice them, as even 6 GB is overkill for quite a number of peoples' usage patterns. Your phone does maybe 95% of what it does just about as well, even when you have a low-midrange CPU and GPU that is from a few years ago, and just 4 or 6gb of RAM.

This holds true for iOS and Android. They've both done a fair bit of housekeeping and software improvements to reel in excessive resource usage gen over gen. I think Android was doing some catch-up here for a while, but I don't know how they go toe to toe on this anymore, and it's difficult to empirically compare the two in this area.