I don't understand why they are different or better than streaming in a comprehensive school.
To answer your question OP, the first point to make is that not all comprehensive schools set, and some don't set for all 'academic' subjects. I'm not aware of any data more recent that 7 years ago, and that is based on Ofsted visits, so could be skewed. However, Ofsted reported then:
Of about 18,400 classroom observations conducted by Ofsted inspectors in secondary schools last year (2008/09), roughly only four in ten represented set lessons:... at secondary, this data suggests that around 53%, 70% and 60% of lessons are set for English, mathematics and science respectively.
So that was a fair proportion of 'academic' lessons that weren't being set. I don't know if that picture has changed, but I can tell you that it hasn't changed in my local comp. (My interest in this issue comes from the fact that my local school doesn't set at all, except for maths, even for years 10 and 11, and my daughter's education has suffered from this.) There is also a problem with smaller schools not having enough top set children to fill a top set.
Personally, I think that if all comprehensives set effectively, and were prepared to seriously crack down on disruptive behaviour, including "low-level" disruptive behaviour, there would be far fewer people calling for grammar schools. But I know from reading many teacher blogs etc over a number of years that a lot of schools have senior management who simply refuse to deal with the behaviour issue, for whatever reason. Although that is also partly the government's fault, for effectively preventing schools from using permanent exclusion as a last resort, and not funding the PRUs and other alternative provision that would be needed if more pupils were to be excluded. While I'm sure grammar schools have some behaviour issues, they are very unlikely to have the same degree of problems typically caused by the most disruptive children in comprehensives.
I'm on the fence with regard to the grammars issue - I believe they shouldn't be necessary in theory, but I do despair in practice of ever getting all comprehensives to be good with current policies on behaviour and setting in particular.