If your house was built before roughly the 1930s, the brick in its walls is softer and more porous than anything sold today, and it was laid with a soft lime-rich mortar on purpose. That mortar was designed to be the sacrificial part of the wall — it moves, it weathers, it takes on water and gives it back, and every fifty to eighty years somebody repoints it. The brick stays.
Repoint that wall with a hard modern portland mortar and you invert the relationship. The mortar is now stronger than the brick around it. Moisture that used to escape through the joints has nowhere to go, so it leaves through the face of the brick instead, and in a freeze it takes that face with it. You will see it within a few years: the mortar lines look perfect and the brick between them is spalling, crumbling and shedding.
Matching, done properly, is four separate matches. Strength and composition, so the mortar stays softer than the brick. Color, mixed and cured on a test panel rather than judged wet, because mortar dries two or three shades lighter than it looks in the mixer. Joint width, which means cutting out cleanly to depth rather than smearing new mortar over the face of the old. And joint profile — concave, weathered, flush, raked, or the beaded work you still find on nicer old walls — struck with the same tool the original mason used.
The preparation matters as much as the mix. Old joints get cut out to roughly twice their width in depth, by hand or with a careful grinder and a hand chisel at the back, never by grinding wide enough to nick the brick edges. Grinder scars along the arrises are permanent and they are the fastest way to make a hundred-year-old wall look repaired forever.
Then find the reason. Joints rarely wash out evenly across a whole elevation. If one section has gone and the rest is sound, look up: a downspout dumping against the wall, a failed chimney crown, a missing kickout flashing, or grade that has crept up over the years and is wicking water into the brick. Repointing without fixing that is repointing you will pay for twice.