This is a standard practice of maintaining your brick structures. This is done when the brick mortar is wearing away. Old Historic brick buildings are the best examples of where tuckpointing is used. It is done on these buildings because the materials on them of course are incredibly old and the quality of these materials are less advanced than what is used today.
You need Tuckpointing if the brick on your fireplace and chimney is wearing out and starting to show its age. Tuckpointing
Repointing/Tuckpointing — If the mortar joints between the brick are cracked or receding, we’ll carefully grind out the mortar, mix fresh mortar, and pack the new mortar into place for a strong, attractive finish. Missing mortar in between joints can be a very simple fix, if caught early on. In Minnesota, the bottom of most masonry garage piers need some tuckpointing as well as your chimney.