It’s a technique used by body builders, and yogis, to strengthen the abdominal wall, and reduce the waist size. Body builders are after that cut, lean, V-shape. Yogis want to improve the functioning of the internal organs.
I have tried vacuuming for many years. First, as part of a daily yoga program. Now, simply because I like the feeling of a tight abdomen. I am more acutely aware of the role the core plays with all movement. On the bike, I realize it is an integral part of the pedal stroke. The core drives the legs. The core is the golf swing. Crunches are not enough. You need to strengthen all of the muscle groups around the middle, and the vacuum gets at those that are otherwise missed.