When the Fantastic Four learned to handle their newfound abilities, they are a constant danger to themselves and those around them. The containment suits are designed to help them harness and control their powers by absorbing and focusing the impact.
The uniforms the Fantastic Four wear are based on containment suits. The team's uniforms are containment and beta suits, designed to stabilize and control each individual's mutation. Johnny's being made of fireproof materials, Sue's being made of materials that can turn invisible with her, and Reed's being designed to keep him together. It's also extremely malleable made with flexor coils. Ben is the only member who doesn't receive a uniform due to the extremity of his own mutation.