Grammar Schools - Why Are They Still Used?

In some areas, it's compulsory to take the a 11 plus. It was one which kind of segregated families and friendship groups. Why is it still used by some? There are always a range of am reasons. One of them maybe som maybe that it simply costs a lot in time, money and lots of other things to change structure. But advocates would argue that if you live in an area where there are many fee-paying schools this has no impact on your education.