Why porous masonry is a problem
As mortar and masonry age, they can absorb rather than repel water, which leads to frost damage in winter as trapped moisture expands and cracks the brick or stone from within.
Breathable vs sealing treatments
The right waterproofing treatment allows the masonry to still breathe and release trapped moisture, rather than sealing it in completely — using the wrong product can trap damp and make things worse.
When treatment vs repair is right
Waterproofing treatments work well on masonry that's otherwise sound; a chimney with cracked pointing or damaged brickwork needs those repairs done first, since no coating will fix an underlying structural gap.