Chop Strand Mat (CSM) is pretty much "Fiberglass Sponge". It offers little strength to the final product, but it does a great job of holding the resin in place.
When you are bonding a core material to the hull, you need plenty of resin in place to bond to the coring material. If you do not have enough resin present, the core material will not properly bond to the hull and delamination will present itself later on in the life of the hull.
Yes, chop strand mat is weak, but its objective in bonding coring is to hold the resin like a sponge so that the core material can be properly bonded and the coring will supply the needed strength to the hull.
If you are bonding two pieces of wood together, chop strand mat can be placed between the two pieces of wood to help hold more resin between them. This will keep the resin in place as it cures the two pieces of wood together. If you use only resin with no chop strand mat, the resin can run and drip out of the void between the wood and a weak bond will develop.
When bonding materials together, chop strand mat works very well to hold the resin in place as it cures and produces a stronger bond, holding the materials together more predictably.