Cowashing is the truth! I consistently notice that I get better retention cowashing than I do with any other type of styling. I had been natural since 2010, and it wasn't until around 2014 when I started cowashing and stopped using heat that I grew my hair past CBL. I think cowashing is good for your hair because:
If you massage your scalp when you cowash, you get daily stimulation. This is assuming you're not using something like Herbal Essences, but your using a conditioner with healthy ingredients. Look for tea tree, olive oil, little or no cones, no petrolatum or mineral oil.
Your hair is ALWAYS moisturized, including when you manipulate it. Manipulating hair in its elastic state can prevent breakage, while your hair is not elastic when it is dry and your are more prone to rough up your ends. It is still a good idea to not brush too much, and to use a wood bristle brush.
I do suggest keeping hair stretched, i.e cowash and bun. Wash and goes can make your hair knot up and give you even more problems.
You MUST MUST MUST deep condition, I can't stress that enough. I recommend you DC at least every time you shampoo, using a plastic cap. It truly makes a difference.