In my experience, schools seem to be asking "what's the benefit of having kids race ahead?" when what they should be asking is "what's the harm of holding them back?"

We're in a situation similar to some others here where DD is struggling with basic math concepts that we were playing with at home for fun in Kindergarten (negative numbers, percents, fractions). Yes, she's allowed to work ahead of grade now and basic algebra concepts aren't hard for her, but her early experiences being held back seem to have given her this idea that math is hard and annoying and not at all fun (and I love math).

"Slow down" is inaccurate. It's actually "repeat ad nauseam until you hate everything about it."