You can leave it on when you actually need it - meaning all summer long, in the conditions that are typical for that location.
It's like saying what's the point of having a house if you aren't safe in it during a tornado. It's an exceptional event. The problem is that these truly exceptional events which only happened once every 10 days for a day or two, are now becoming a norm. My own 12k BTU minisplit could cool my house down every summer, no problem, even down to 18C if I wanted it to. But in the recent heatwave it just can't keep up, there is more heat coming into the house than the system can remove.
It's like saying what's the point of having a house if you aren't safe in it during a tornado. It's an exceptional event. The problem is that these truly exceptional events which only happened once every 10 days for a day or two, are now becoming a norm. My own 12k BTU minisplit could cool my house down every summer, no problem, even down to 18C if I wanted it to. But in the recent heatwave it just can't keep up, there is more heat coming into the house than the system can remove.