It's necessary to maximise the weight lifted. Some force is transmitted through the trunk as opposed to through the spine and back muscles. The organs are broadly speaking incompressible, but the lungs are not.

Squeezing the abs pushes the organs up and holds them in place, compressing the air in the lungs, creating a more rigid trunk. This stops lifters from having their chest drop down, which will cause a dropped lift or even an injury.

Of course, if you squeeze the gut, the stomach can decide it's time to vomit if you have a lot of food in it. So ... don't do that.

Similarly if you're having lower intestinal disquiet.