It helps prevent hydral fatigue.
From Science-y Hair Blog:
" 1) Oil pre-shampoo! This is one of the best times to use oil,
even if it seems counterintuitive to put oil on your hair before you're going to wash it - in part to remove any excess oil. Apply a light (or heavy) oiling to your hair and leave it on for 2-12 or more hours before shampooing or otherwise cleansing your hair. The oil prevents dehydration and loss of proteins and otherwise protects and buffers your hair from water and detergent. If you get the amount of oil and leave-on time right, your hair will be more flexible and feel softer, better lubricated and if you have waves or curls, they will be better-defined. The longer you leave the oil on, the more oil will work its way into your hair. "
"Here's the deal:
You know how washing your hands too often makes them chapped - even if you don't use soap? Washing your hair ultimately dehydrates it too unless your hair is tremendously low porosity. By that I mean the cuticles are un-damaged and in such pristine condition that they don't let water in at all. Porosity is a surface thing. Oils have other great effects like making hair soft and flexible - those are partly related to porosity. Even low porosity hair can get dried out with too much washing or dry air or a lot of sunshine.
Any oil will keep the water out of your hair for a little while and that is great. Many oils can surround any chipped and broken places in your hairs cuticle covering, giving you better protection.
Hair-penetrating oils are ideal for some people's hair because they "waterproof" the inner part of your hair that swells. If we can keep water from getting into our hair and swelling it up, it won't end up waterlogged and the cuticles will chip off less and we'll keep our lovely proteins and everything that keeps out hair strong from escaping down the drain.
My top picks for oil pre-washes:
Coconut oil, sunflower oil, babassu oil, olive oil, avocado oil, a blend of these oils and any others. Castor oil is a distinct "maybe." "
I have plenty of moisture in my hair already.
What would that be?
It's pricey, especially because I have been reaching for it to seal recently, but it gives my hair this really strong feeling. And I swear the regular henna use has increased my strand thickness. I'll be investing in a few more 16 oz jars Black Friday anyway. It's a staple for me until I can dupe the recipe.
For my hair it's mostly no new tools/techniques, no new extreme products that may cause a set back (i.e., a new protein treatment), ends tucked away as much as possible, use my DIY scalp oil on schedule etc. Stuff you're supposed to do anyway 
