Water is the enemy of masonry! This sums up everything you need to know. Control the water, save the masonry.
Most situations do not require masonry sealing products to keep water from causing damage, but there are situations that do benefit from sealing. Masonry must be able to “breathe”. A proper sealant needs to block liquid water from entering the masonry and allow water vapor to escape. The proper products are like an invisible Gore-Tex for the masonry.
An acceptable product is siloxane because it’s waterproof and breathable. Its downfall is that it is UV sensitive meaning that it needs to be reapplied every 5 to 10 years. The best product is MasonRE R97 Water repellant by Cathedral Stone. It has a higher degree of breatheability than siloxane and is UV stable so it does not need to be reapplied.