To ensure optimal brake performance, it is always ideal to replace brake discs and pads together whenever either item is due.
However, in balancing perfect performance against cost, you do not always have to replace both together.
When having new brake pads fitted, if the brake discs are still in good condition, it is not essential to replace them too.
However, when you are having new brake discs fitted, you should always replace the pads at the same time. Failure to do so will often lead to brake squeal, and more importantly, the old contoured and often ridged pads can damage the new discs, potentially leading to the premature replacement of both components again.