The short answer is this: there is no benefit to having more RAM than you need. Having less RAM than you need is a disaster.
Basically, the moment you run out of RAM, your processor has to start swapping things in and out of memory by sticking them back on the hard drive. Not only is the hard drive much, much slower (even an SSD), but the swapping takes time as well. Think of it as RAM being the size of the desk in front of you and every time you run out of space on it, you have to take some of the papers on it to the filing cabinet in the next room. And then when you need them again, you have to take something else that's currently on your desk and put it in the filing cabinet so you have room to get the first thing back again.
That's a simplified version. Modern OS's also use RAM for caching things from disk to save reads and as a buffer for writes. This means that excess RAM is not a complete dead weight, but the benefit of this is not usually that large and there are limits even to how much it is useful to cache.
8GB is more than most people will ever use. People doing heavier image manipulation or running a VM might want 12GB. Beyond that, you're looking at specialist needs. People rendering complex 3D scenes (note, not games, graphics work), people doing less common database work like me or those running multiple VMs. That sort of thing.