The number of subjects your child has to study for O level is significantly more than what they have previously faced in PSLE.
This means that a good grade in PSLE
will not be a good gauge of how your child does in their O levels.
The O'levels are very important because regardless of what the school teachers, it is a Cambridge exam after all and you can't be certain that your child's tutor has spotted the questions correctly.
A tutor offers a third perspective on what additional topics that your child's school teacher may have missed out.