Because you cant run as much timing with out getting det and cold air is more dense so you make more power per a psi.
A few degrees of intake charge has a large effect on air density, and a very minor effect on charge heating. 49% of charge heating takes place when the intake charge strokes across the superhot intake valve, 49% takes place on the compression stroke (you compress shit it gets hot, y0), and the other 2% is dictated by other things most of which aren't IATs.
When doing IAT trims on Hondas I mainly pay attention to fuel corrections. The ignition side, meh, that stuff is mainly there to advance timing when intake charge/manifold is super cold so that the car idles correctly. The only time I touch the "hot" IAT trims is to remove ignition retard in the 130+ degree F range, as the only thing that consistently runs that hot is a blower car and they don't move enough air to have the small amount of relatively hot air affect ignition timing requirements by speeding up the burn. At all.
Things might be, and quite likely are, different for some other platforms that aren't Hondas. But I'm confident those are exception to the rule type situations and not the way shit works. 90% of my tuning experience is with Hondas, but I've tuned a large pile of cars in the last decade and I walk into every tune with my ears perked all the way up; so far it's worked out that way.