This may just be a personal bugbear but when training at the swimming pool it completely amazes me how many people I see swimming who don’t push off the wall or dive into the water in a streamlined position.
Diving into the water and pushing off from a turn are when you will be moving faster than during any other part of your swim. Ironically this is also when you are exerting the least amount of effort.
So in essence this is ‘Free speed’. It’s free and it’s fast, so it seems silly to waste it.