It's dangerous to assume that everything in biology exists because it's useful in some way. Some things are just spandrels* that came along for the ride, vestigial, or otherwise neutral features. Not everything exists because it provides an evolutionary advantage.

*: https://en.wikipedia.org/wiki/Spandrel_(biology)

I don’t believe so. I looked into it a bit more, and it looks like homozygous variant carriers are more susceptible to other types of infection (West Nile is specifically cited), but I don’t see anything showing the variant causes issues with CCR5s other critical functions. I do think a knockout is probably a bad idea based on further reading, but modification seems like a promising path.