To build a big background, you can't build it without a truss. The wedding I am in charge of, the biggest one, has to be built 5 meters high in the background, and the trusses have to be reinforced for fear of falling.

In addition, if you want to build a big flower gate or flowers, you need a truss. Ordinary arches can't add so many flowers.

Generally speaking, because the truss is relatively stable, if you want to make a larger background, flower doors, decorative frames, etc. You need a truss.