Water is usually the enemy...chimneys included!
Updated: Sep 22, 2019
Chimneys: lofty, solid...out there in the wind and snow and rain. But like all of us, vulnerable. Brick work isn't designed to act like a roof (like in the sloped parts of chimneys) or dams, when the sides of chimneys meet the roof. Hence the poetically named cricket diverter, which is made of metal or roofing material and sends the water, snow and ice to either side into the gutter. Otherwise, ice can form a dam uphill of the chimney and water pools against flashing which deteriorates and harms the framing of your house. The requirement is that any chimney more than 2 feet in width have one.
Here is the fanciest one I could find. Click on image to get a whole gallery of choices.