I started using I joists back in the 90's because of the strength and evenness of the floor. And houses got bigger, and we needed longer spans for the homes people wanted to buy. With lumber floors, we always had gripes about squeaks in the floor, I got to using screws to fasten down the plywood, and it helped a little. But I joists got rid of the complaints, as they are so even on top, the floor is flat, so no low joists where the plywood pulls the nails up.