This is particularly important at the initial state where you're eroding down to a level - when you're just going through grits smoothing it matters somewhat less.
https://youtu.be/WtExoSMgvXI?t=260
These guys are sanding down the mobile CPUs and soldering jumpers to do a password reset. I didn't know this was even possible.
Much easier when you can let a power tool do most of the work.