Because you can. And really because they really can make shooting more enjoyable. Due to some heavy-handed regulation in 1934, suppressors, short-barreled shotguns and rifles are limited in ownership through strict regulation and there remains an outright ban for civilians on machine guns manufactured after 1986.
Suppressors are the first logical jump into owning NFA items. It is easier to keep shooting fun when it becomes less about taming recoil and noise and more about hitting targets. When hunting, suppression allows you and fellow hunters to not wear ear muffs and remain attentive to both game and one another.
An interesting note about suppressors: In other parts of the world, even countries where firearm ownership itself is highly regulated, firearm mufflers can be available for purchase over-the-counter at hardware stores. Un-suppressed shooting is even considered obnoxious when the solution is so affordable and readily available. There are currently legal efforts underway to align United States law so that the financial and time burdens of the tax stamp process is eliminated.
Owning short-barreled rifles (SBR) and shotguns (SBS) follow much of the same logic as silencers. Because you can, and because there are some things that a shorter rifle or shotgun will do best. Perhaps you are trying to get your home defense or competition rifle at the perfect length, particularly when using a suppressor. Maybe your want a handy and suppressed slug-gun that your kid can use. If you are noticing that pairing multiple NFA items is both practical and increases cool-factor, you're catching on.