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.