A fume extractor is at heart a simple machine. Dirty air is sucked up at the fume arm (snorkel), drawn through to the filter, and then clean air passes through the fan and out the exhaust.
Though simple in concept, the craftsmanship and knowledge required make it more difficult in execution than it would seem. Different particles (fume and dust) require different filters of varying degrees of efficiency. Getting the ideal airflow so that the fume can be extracted from while not pulling the shielding gas requires careful consideration.
At the end of the day, a weld fume extractor sucks dirty air in and exhausts clean air so you don’t inhale dangerous, cancer-causing fumes.