Spring seats (or spring perches) are welded onto axle tubes which allows your leaf springs to sit flat on the axle. When the leaf springs are sitting on these seats, you can securely mount your axle to the leaf springs with a u-bolt kit.
Your trailer requires 2 seats per axle.
Spring seats for round axles MUST be welded to your axle otherwise your axle will spin when your trailer is in tow. Improper use of these seats can lead to immediate issues and damage once you begin towing your trailer.