Rate of force development is the speed (time it takes) to produce force. Those that have a high rate of force development can produce force faster than others. This is obviously important for sport and performance. In sport you only have so much time to produce force, things happen fast. So, if you can produce force rapidly, then you obviously might put you ahead of those that produce force more slowly. However, we do need to understand that the rate of force development is still dependent on the amount (max) force you can produce. Having a high rate of force development, but not producing much force might not be as beneficial as having a medium rate of force development, but being able to produce lots of force (high max force). Why is that? Well, if you think about it in terms of magnitudes, it starts to make a lot of sense. Say one person can produce 200 lbs of max force and the other can produce 400 lbs of max force. Now, let's say that person who can produce 200 lbs can produce 100% of their force in one second, but the person who can produce 400 lbs of force can only produce 75% of their max force. Who can express a higher percentage of high force, obviously the first person. Who can produce more force? Well the second person. You see, 75% of 400 lbs is 300 lbs, which is a lesser percentage of force compared to the first person but a higher overall magnitude 300 lbs to 200 lbs. This is why force production is the base of all other strength qualities and solely focusing on aspects like rate of force development might leave you lacking in other areas.