Mortar joints fail before bricks do, and crumbling mortar lets water into the wall where it causes worse damage. Repointing — replacing the old mortar — restores the wall and is doable for a patient DIYer, though matching old mortar takes care.
Rake out the old mortar
Remove failed mortar to a depth of about twice the joint width using a chisel or grinder, taking care not to damage the bricks. Brush out dust so the new mortar bonds.
Match the mortar
On older homes, matching the mortar's strength matters — modern hard mortar on soft old brick can cause the brick to spall (flake). For heritage masonry, use a softer lime-based mortar appropriate to the era. Match the colour and joint profile too.
Pack and tool the joints
Dampen the joints, pack new mortar in firmly in layers, and tool the joint to match the original profile once it's thumbprint-firm. Keep the new work damp as it cures so it doesn't dry too fast and crack.