People I know have done it for various reasons a) because they want their children to get all A* grade b) because they don't want their children mixing with the people at the local comprehensive and c) because they think it looks good to have children at a private school.
If you send your children private do you mind saying why? I'm genuinely interested (and I'm not a journalist). We didn't choose private schools because we thought our DC would not be comfortable being amongst the poorest families there.