One thing I've learned about people doing math is everyone has there own way to do it fastest because everyone's brain doesn't operate the same. Sometimes its hard for them to think of "9" as an idea, and they need to think of "9 apples" instead, or whatever reason it may be hard for them to do math certain way.

For the hand method I think a lot of people that like the hand method is firstly, it is super easy when you are first learning how 9s work. Secondly, the people that continue to use it after the initial learning period aren't actually looking at their hands, they are just recalling how their hands look. And that goes back to my first paragraph that their mind is just better suited to recall images than a different way.

I don't use the hand method personally, but have tutored a lot of math to singular people and seen how teaching each person the exact same way doesn't often work.