Hi @LN! The intent is really just to anchor the slider somewhere other than extreme right or left without that placeholder being taken as a meaningful response to the question. If you use the default to anchor the slider in the middle of the scale, the respondent can skip the question(s) without moving the scale and it would appear as if an intentional decision was made.
I don’t think it has to do with biasing the value or that it’s hard to see the slider. I agree that placeholder is a more intuitive naming since it’s a value that’s intended to be replaced by the user.