When you're riding your bicycle outside, wind resistance increases exponentially according to your speed. It's more than twice as difficult to ride at 20 mph than at 10 mph. The best stationary trainers offer built-in progressive resistance: pedaling faster or shifting the gears on your bike results in increasing resistance. With linear resistance, you lose this road-feel. Pedaling faster or shifting your gears has little effect, other than spinning your rear wheel faster.