The blood brain barrier prevents many substances from entering the brain. The brain has far less tolerance for many substances. The blood brain barrier protects it from these substances/toxins. The myelin sheath protects the axons. It also houses the glymph system, a system similar to our lymph system that carries toxins away after metabolism.

Would you prefer to drive around in a car with dirty oil and dirty gasoline or one with clean oil and clean gasoline ?

btw, Buildup of toxins in the brain are a big part of depression and other mental struggles.