What failure looks like
Sagging or stained soffit panels mean water is getting in above, usually at the gutter, the drip edge or a failed roof detail. Replacing the panel without fixing that just conceals it. Soft fascia is normally rot, and in Florida frequently accompanied by carpenter ants or termites in the timber behind.
Ventilation
Vented soffit is how air enters the attic. Blocked by insulation, painted over or replaced with solid panels, the attic stops breathing, and in this climate that means heat build-up and moisture on the underside of the roof deck. It is a small detail that affects the roof, the air conditioning bill and mould risk.
Why soffit and fascia repair is rarely just carpentry
Because the board is a symptom. Rotted fascia under an overflowing gutter will rot again on the same schedule unless the gutter is resized or the roof edge detail corrected, and blocked soffit vents raise attic temperature and moisture enough to shorten the roof above them. The ventilation requirement itself is in the Florida Building Code.
Material
Aluminium and PVC both resist rot and are the sensible default. Timber fascia is fine if it is kept painted, and it will not be.