Most folks blend RO to get a desired KH, the GH is generally high enough still, and higher than the KH in most cases.
So if you knock the KH down to 3, from say 10, your GH is likely 10 or higher as well, so you end up with 3GH etc also.
Some folks go 100% RO, not sure why, not good reason unless there's really high copper or something really bad.
Some like to suggest so they 100% sure of the make up water.
Well, I guess, more work, wasted water, add the stuff back you just took out, no need to drop the KH below 3 really etc