Why the whole roofline gets done at once
The fascia holds the guttering and protects the ends of the roof timbers, and the soffit closes the gap underneath. When one of them fails the others are rarely far behind, so patching a single section tends to mean calling someone back in a couple of years.
Replacing the timber with uPVC and fitting new guttering while the roofline is exposed means it all gets done in one go. uPVC doesn't rot and doesn't need repainting, and a black fascia and gutter line gives an older house a sharper look against the brickwork.