Say it's 25C outside and 30C indoors. I want to bring the indoor temp down to 25C, but opening all the windows barely makes a dent. Does it make sense to have the AC turned on and set to 25C while I have the windows open? Or should I be closing them anyway?
As long as having the windows open and the AC running has the same effect, it makes sense to do both. As soon as the AC has to work against the air coming in from outside, it probably makes sense to either close the windows or turn off the AC.
Even then it might make sense to keep doing both though. For example, if the AC only changes air temperature and moisture, but does not reduce volatile organic compounds, and it's hot and humid outside. You keep the window open to keep VOC low, while keeping the AC on to keep temperature and moisture pleasant.