More modern ballast technology uses digital circuitry to control power, making them more efficient and less bulky than magnetic ballasts.
Typically adjustable to different wattages, digital ballasts allow you to run lights at lower power. This can be helpful during early veg or high temperatures, whilst higher wattages give your plants the full power of your bulb. Most also provide a boost function to boost power by around 10%, so a 600w can be boosted to around 660w, and a 1000w to 1100w etc…
Digital ballasts run much cooler as well as being more efficient. They are silent running and have improved safety features compared to magnetic ballasts.