Chicago chimneys are exposed to very heavy weather effects - strong wind, very cold/snowy winters, rain - while in the same time they are heated by hot gases from fire. That can cause a lot of deterioration, cracking and movement to your chimney.
Repairs of your chimney may include repointing or rebuilding the firebox (where fire is built). Sometimes the damper needs to be repaired, so it can close properly the flu so heat wont escape through the chimney.
The mortar crown is the concrete or stone surface at the top of the chimney that keeps water out of flue. That part of your chimney should be repaird immediately when it's cracked or broken - so you canl avoid future serious water damages and huge repair costs. If you will leave it unrepaired it will provide the eay for water to penetrate the masonry. Then freeze cycles will casue masonry deteriorate. Unrepaired can release dangerous gases inside your home or even flames and sparkes that can create a fire hazard.