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