Fisheries managers use slot limits to manipulate the growth level of fish. They basically want anglers to harvest the smaller fish (that tend to be eating size anyway) in order to offer more food for the fish in the slot to grow into trophy sizes.
Slots offer more potential to catch that fish of a lifetime while thinning the herd of fish that are growing into the slot. They need to be carefully monitored and regulated to ensure a balance of the fish population.
As long as that balance is maintained, it's good for the fish, and GREAT for fishing. Dr. Hal will tell ya that.