Sweatshirts are often used for aquatic boot camps, where the training is hard because you're always fully clothed in the water. This builds muscles, strength and stamina.
Sweatshirts and sweatpants soak up more water than their own weight which you have to move along in the pool or on poolside, giving you a better workout because of the added resistance.
If you teach an advanced swimming class that involves games or team sports, sweatshirts are a good way to dress in different colours to identify swimmers in team sports and games. This is great fun and builds team spirit.