Pace (minutes per kilometer or mile) is how most runners think about effort. It connects directly to race times and training plans. If your target marathon pace is 5'00"/km, you know each 5K split should be 25:00.
Speed (km/h or mph) is what treadmills display and what cycling, swimming, and other sports use as their default unit. Most running watches default to pace, but treadmill runners and cross-training athletes often need to convert between the two.
Neither is better. Pace is more intuitive for planning race efforts. Speed is more intuitive for treadmill settings and comparing across sports. This converter handles both directions instantly.