the point of seam sealer is.....well.... to seal seams basically lol.
it puts a stop to debris over the years from falling into the seams and causing a spot that holds moisture which causes rust. stops outside elements from getting in. can help cut down on road noise and maintaining interior temperature (depends on how big of gaps and location of gaps of course) . helps make for a cleaner look once its all in place and painted over.
My dusters factory interior seam sealer was horrible....it was effective but ugly lol. from the factory they just goobered the stuff all over and in large clumps.when you do it you want to make a clean job of it.