Think of it like a neuron is your house and the blood is a Highway. We need the things traveling on the highway (like people, Amazon orders, etc) to make the house function. But there are a lot of other things on the road that if they directly contacted the house (like a 16-wheeler moving at 60 mph) would cause damage. Our neurons need the oxygen and nutrients, but sometimes the things that bring the oxygen (like hemoglobin which contains iron) can be toxic if they are released onto bare neurons. That’s why neurons are separated from direct contact with blood by other cell types like astrocytes and pericytes which more carefully regulate how neurons get what they need from blood.