You need a dust collector for two reasons:
First off, even working outside, you can't avoid breathing in airborne debris, some of which may be toxic. With the right dust collector and a well-maintained filter, you can see to it that your work environment is not a hazmat zone.
Secondly, a collector allows you to recover and recycle your scrap easily and profitably. Phil Scott, of the Rio Grande Jewelry Tech Team (who owned his own shop and still does custom work in his off-hours), tells me that he sent his sweeps to a refiner about every quarter. He says, "If I'm polishing silver or gold, I collect everything. Removing scratches means removing metal. It has to go somewhere, and if I leave the dust on the floor, it's like I'm tracking dollar bills right out of my shop and throwing them to the winds. The better my collector is, the more of that money I eventually get back. Depending on how much material you process, a good collector could pay for itself in just a few months."
To choose the best collector for you, consider the amount of work space you have, what processes you're doing, how much volume you handle and what your budget demands.