I think it’s little difficult to add a new statement to this conversation, as everyone here has made very good points, but here is my 2 cents.
The monitoring and controlling phase are seen throughout the project to evaluate risk, and lower cost. We know that the PV and AV along with the EV dictate either a positive or negative SV and CV. I think it is important for the monitoring and controlling phase to look after these numbers to make sure you are staying as positive as you can. Having an idea about where the money is, you can understand what resources you need to use to keep it on schedule. Without their help the budget would be all over the place.