Think about it this way. Your face is given a lot more attention than the skin on the top of your head. A facial makes the skin on your face feel fresher and cleaner. It's the same for your scalp.
It might not seem that logical, partly because most people have a thick head of hair, but it's the reason why conditioners and hair oils have been used for as long at they have.
Without these, your scalp can feel rough and dry. Moisturizer on your face helps to eliminate the same kind of feeling.
Scalp facials can help to stimulate hair follicles and keep pores cleaner. Anyone who has had a scalp facial will be able to say how amazing it is. It's like your scalp can breathe again. You feel fresher and relaxed.
I've been using sea moss gel on my face, head and decolletage for years (exactly how many, I can't recall). This has helped my skin to feel firmer, more hydrated, and look younger.
Whatever you do use for your scalp facial, make sure it is natural. Avoid nasty chemicals as much as you can.