How old is your dd and how long has she been dancing? For recreational dancers it is nice to actually have proof of your achievements, and for parents proof of money well-spent.
It all depends on whether you are dancing as a hobby or if there is an expectation of a professional career. Some dance teachers and schools insist on exams and won't let you move up a class until you have taken the exam anyway. I don't agree with this, by the way.
When you are audtitioning for a place at a vocational dance establishment for full-time training, they assess the dancer performing in front of them, they aren't looking at pieces of paper. To get an audition for a first professional contract, they are either open auditions where you just turn up (and again, are assessed on the day) or closed auditions (where you apply in advance and they call you in based usually on where you trained).
So exam passes are not a requirement career-wise. Some application forms ask which grades have been passed, but they know that not everyone takes exams so you can put which level you are currently studying/performing at instead.
Having said that, in order to enter some competitions (such as the Genee) you do need to have passed exams at the required level.
When it comes to taking later exams if you haven't passed earlier ones, for ballet grades up to grade 8 (depending on exam board) you can take them without having passed the previous ones. Some people start taking classes as older children, so it would be ridiculous for a child of 12 to have to join a class with 5-year-olds. They would start in a higher grade.
For vocational ballet grades, however, then you do need to take (for instance) Intermediate before you can take Advanced.