I don't think I've heard Coach O acknowledge that, so this may be nothing more than my opinion, but I think that's why.

One of this team's weaknesses is its passing. Passing sets up good sets, which are required to run your offense "in system" and get the hits you want to attack the other team's weaknesses. I think that Robbins-Hardy has better passing on serve receive (less important because she should be taking the second hit), and I think she also has better blocking (she is taller and more athletic) and better lateral speed and digging. So, better defense.

The games BYU has lost, BYU has lost largely because of their passing and digging. Coach O would put her in not to cause us to lose, but because we were losing and he was looking to give the team more help in those areas. Her setting, truth be told, is probably not quite as solid as CWGs, but it is good and will get better. Part of that is a timing thing and part of it is the setting technique itself. I think Robbins-Hardy's placement and decision-making is very good. But as Robbins-Hardy's setting has gotten better over the course of the season, her overall skill set has made her the better go-to setter.

Interestingly, Robbins-Hardy was recruited as and came in as an outside hitter, not a setter. She has learned the setter role this year. I think that explains both why her other skills may be stronger and why her sets are not quite as solid as CWG's.