Why use a wash bone like this, instead of a mitt?
Sponges can compress dirt against your paint, so covering them in deep pile microfibre to create a 'wash bone' removes this danger. You then get the benefit of that absorbent sponge core - metering out wash water to sluice away dirt. They can be easier to clean than sponges too, because dirt won't migrate into them so far. The only disadvantage compared to a mitt is that they need to be held the whole time; you can wear a mitt but you have to hold a bone.
Can it scratch paint?
Not if used correctly and kept clean. Rinse it frequently to release trapped dirt, and wash it fully after use.
Is it machine-washable?
Yes, give it a good rinse to get rid of large pieces of debris, then put it in with your microfibres (30-40 degrees, non-bio liquid or dedicated detergent like Furry Liquid, NO fabric conditioner or softener... tumble dry at low temps after).
How long will it last?
With proper care, a wash bone may outlast a mitt as there's no cuff to stretch. Don't leave it soaking in a bucket after use - this could rot the stitching and shorten its lifespan.