Because if you dont, then you could run your car up on a very cold day and get a massively inflated figure that it wouldnt be able to adjust back down, likewise the opposite on a hot day.

And because if you build two identical engines and roller one on a hot day and one on a cold day they would get totally different figures.

How the hell is that useful for developing an engine?

Any engine from december would seem massively better than any engine you built in july!