Temperature is a parameter that is fundamental to the physics of transistors, analog and digital integrated circuits, and other semiconductor devices. Consequently their behavior varies significantly as a function of temperature, and a competent designer will design the product to perform at its best when it has warmed up to a stable internal temperature, while being used in a room that is at a normal room temperature.That's about as layman an explanation one can give, and it was his first paragraph.
All the best,
Nonoise