On farm ground, increased traction is part of their purpose, but decreased soil compaction is an equal or larger part. You can add significant weight to a tractor with single tires and get fairly good traction, but it will cause severe soil compaction. Adding duals will slightly decrease slippage, while drastically decreasing compaction. Soil type is also part of the equation when figuring the weight to footprint ratio.
I didn't know all that when I was 12. I just wanted my tractor to look bigger. That is the same reason I'd like duals on my 446. They would be more of a hindrance than a help for my purposes, but it would look cool. :thThumbsU