Penetrating sealers are the most common sealers used on bricks. These sealers clog all the pores within the masonry, which keeps it from absorbing too much water. They are translucent and do not create a film. When applied correctly, the sealer looks as if its not there at all. These particular sealers also allow moisture trapped within the masonry to evaporate rather than locking it in -which would cause further damage. Internal moisture is usually caused by heavy winds forcing their way through hairline cracks in mortar or bricks.
If the masonry is not cleaned and prepared properly, or if the sealer is not applied properly, then the sealer will not penetrate and fail. At the very least, the sealer would last only a few years before the masonry starts absorbing moisture from the exterior again.