Seams are always the weak point in any gutter system. No matter what you do, there will always be some movement at the seams, even if the motion is too slight for you to see.
That movement weakens the seals, allowing rainwater to leak through and make the seal even weaker. As the process continues over the years, the structural integrity of your gutter system gets worse and worse until it fails completely.
With seamless gutters, each length is customized to fit your roof edge perfectly. The rigidity of a single extruded length of gutter is far superior to one consisting of multiple lengths pieced together.
And without all those seams, there’s nowhere else for the water to go except to fall neatly down the spout.
Also, seamless gutters simply look better on your home. Their straight, unbroken lines make for a cleaner look than traditional gutters.