Why should I use a fume extractor instead of wearing a respirator?

The short answer is that a respirator protects the welder wearing it, but does nothing for nearby workers not wearing similar protection.

The somewhat longer answer is that a repirator, as good as they are in protecting the person wearing one, does not capture and contain weld fumes in the general atmosphere.  As such, other workers in the area could potentially be exposed to high levels of hazardous welding fume particulate.  A fume extractor can mitigate that risk because it captures fumes from the local area of welding, which is why fume extractors are often referred to by the the term Local Exhaust Ventilation.

In critical work applications, or wear hazardous exposure levels are high, repsirators AND fume extractors may be employed in combination with each other.  Your safety manager and/or an industrial hygienist can advise you on best practices for the reduction of exposure to hazardous welding fumes.