Why Should I study Physics and Maths together at A - level? Watch
If you're asking why you should study both physics and maths rather than just one of them without the other, it's mainly because A level physics requires you to be very comfortable with manipulating equations and doing calculations and whilst the physics course is designed to be accessible to people not taking A level maths, the extra skills you pick up when doing lots of maths can be helpful in physics. Many people who take physics without maths at A level tend to do far less well than those that do take maths, although other factors (e.g. interest or future uni plans) may influence this as well.
If you're planning to study A level maths, there's no need to do physics alongside it if you don't want to, although a very small number of the top universities like it if maths degree applicants take physics.
Also, degree programmes involving physics (or fields like engineering that use some applied physics) virtually always require A level maths alongside physics, so you wouldn't be able to apply for one of these courses without A level maths.