If they've come from abroad them they will not be familiar with the curriculum and may well need support.
If they are at a London prep they will be doing French and Latin which may be new to them.
English may not be the children's first language.
Parents obviously here for work and the salary offered suggests parents have high flying jobs therefore they may not be able to spend as much time supervising prep as they would like.
I'm sure there are many reasons why these parents want to support their children as they transition into the UK prep/public school system.
Or they could just be the type to outsource their childcare.....