Why use a bat warmer?
Have you ever swung a bat in really cold temps and had the bat sting your hands? This is a result of the bat's fibers becoming stiff due to the cold temperature. Bat warmers can make them flexible again. Bat companies even tell you not to swing their bats in temps under 65 degrees or it will void the warranty. There is also a very noticeable increase in the bat's performance when it is warm vs when it's cold.
We have compression tested the same bat at 45 degrees and at 80 degrees and there is a 200 PSI change in the compression reading. That means that your bat is much more flexible when warm which makes it less likely to break or sting your hands and will most definitely make it perform better. A 200 PSI compression difference is almost like comparing an ASA bat to the Miken Ultra senior bat.
There are two reasons why bats break more often in cold weather. One is due to the fact that softballs change compression in cold weather making them much harder and are thus tougher on your bat. The other reason is that the bat's fibers change compression and become stiff and inflexible which will give it a greater chance of breaking. Having a warm bat gives you an extra layer of protection against the bat breaking than if you swing it cold.