What the word "system" is doing in "GAF system"
A shingle is a water-shedding surface, not a waterproof one. It is designed to hand water down to the shingle below it, and as long as water is moving downhill across an unbroken field of them, almost nothing goes wrong. Roofs do not leak in the middle of the field. They leak at the edges and at the interruptions — the eaves, the rakes, the valleys, the ridge, and every hole cut for a pipe or a vent. Which means the parts that keep the house dry are mostly the parts you cannot see once the roof is finished.
The starter strip is the first of them. A manufactured starter puts a factory adhesive line exactly where the wind wants to lift the first course, and gives the shingles above it a straight, sealed edge to bond to. The old shortcut is to cut up field shingles and use those instead. It looks identical from the ground and it holds fine — until a windy night, because the adhesive strip on a cut-up field shingle ends up in the wrong place or upside down.
Above that: leak barrier, the self-adhering membrane at the eaves, the valleys and the penetrations, which seals around the nails driven through it and holds back water that backs up under ice. Then underlayment across the rest of the deck, keeping the deck dry during the install and giving the roof a second plane afterwards. Then ridge cap, which is a purpose-made piece, thicker and formed to bend over a peak in cold weather without cracking. The substitute there is three-tab shingles cut into thirds, on the single most weather-exposed line of the whole roof. And then ventilation, which belongs in the list because the manufacturer's own numbers on shingle life assume the deck underneath is not cooking.
Taking all of those from one manufacturer is not brand loyalty. It is that they have tested those pieces against each other, and they will only put their name on an assembly they specified end to end. Accessories are a small share of what a roof costs in materials, so swapping in generic ones saves a little — and quietly converts a covered system into a roof with a shingle box and nothing tying the pieces together. That substitution is invisible from the driveway and permanent once the roof is on.
The practical version for your own house: when you are holding two quotes, read the accessory lines. If one names the starter, the leak barrier, the underlayment, the ridge cap and the ventilation, and the other says shingles and accessories, you are not comparing the same roof — and the cheaper one is cheaper for a reason you will not be able to see once it is installed.
