If I only dyno'd my car in the summer I'd only know how it performs for about half the year. Where I live the other half is cold. Now in Florida or Socal it's different, but in New Jersey we drive around in freezing air during the winter.
Just like if someone who lives in Denver dyno's their car at sea level. That isn't accurate because they spend 99% of their time at the higher altitude where their engine puts out less power. It's good to know how your car performs where you spend most of your time.