From experience, I have known "lower ability" children to not be as low as first thought. The topics within a subject are very vast. This is the problem with setting as some of the lower ability kids can surprise you, depending on the lesson or topic.
Children who are generally poor at maths, can actually be very good at money or certain areas of maths. I taught a kid, who had a very tough home life, and amongst other things, did the cooking, was sent to the shop to buy milk etc and was very poor at maths (times tables etc as he never did his homework). However, when learning about money in maths, he was top of the class and in this area was very much "high ability". Similarly with measure, he was excellent as he did lots of cooking and weighing ingredients! And this is where the problem is with putting children in high or low sets. He was brilliant at money, because ofcourse he was using money at in his life to do the shopping etc. He would otherwise been put in the bottom set, when actually in this area he was better than the other kids.
"Low achievers" can also be excellent at PSHE, Geography as quite often, they live chaotic lives, travel a lot, sometimes more streetwise than the "academic" kids. Or what happens is, the kids labelled as low achievers in English, often get put in the low sets for all the other subjects, when actually some of the lower kids in English, end up better at Art, Geography, RE than the top kids.
So what's in it for the "high achievers"? You can teach a class with a mix, and adapt the learning for all. The children who need to be challenged, can be given more challenging questions than the others in the same lesson for example. They also benefit from explaining their thinking, when helping a low ability child. Talking learning, benefits all involved.
Setting is now old news and the reason is that to label a child as low ability, is just not right. As I said, it just depends on the topic. There are examples of course of children who always need extra support, but having all abilities in one class, enables more success for those who otherwise would have been given easy work in the bottom set each day.
That is not to say though, that it's easily done. The teacher has to carefully work out which child should be given which questions, according to their ability in that particular topic. It's very hard to get right, it's a lot of planning and work for the teacher, but it's true that there is no such thing as a low or high child.