It's a major arse-covering exercise in the wake of the Soham murders - virtually everyone who sets foot in the school has to be checked, and we need a great big fat visible file to wave in front of everyone to prove that we've taken every step we can to protect our children. There's a List 99 search too, which lists everyone who committed a crime while working in an educational establishment.
There's too many holes in it though, to be of any real use. There a 6-8 week delay in getting the details (plenty of time to go out on the rob) and we are only obliged by law to have it done every 4 years. It doesn't cover anywhere outside Europe in the main, and can't handle cultures with multiple names. Also bearing in mind that the vast, vast majority of child abusers have never been accused, let alone convicted, it's a bit of an empty exercise.
The only value it might have is to deter those with convictions...trouble is that it probably deters those with little tiny convictions for nothing very much, and they might be the sort of people we'd like to have.