When separating the frames, the outer has 2 bands of sealant - one next to the glass and the second next to the door. If you are not careful when removing the frame from the door or the glass from the frame, these bands can be damaged. I stretched a few bits on mine, but then pushed them back into place or trimmed as necessary. Before putting the new glass in, I ran the thin bead of water based sealant between the band and the edges to create a better seal than the original.
I can see where it is a good idea to replace the frames as this seal material is usually destroyed. Therefore the kit "fixes" this issue - new frames and glass.
