If daily cleaning is brushing your teeth then Deep cleaning is flossing them. While brushing teeth daily is important it is insufficient if not supplemented with regular flossing.
Toilet and urinal pipes build up deposits of uric acid scale, faecal matter as well as normal minerals and salts found in water (such as limescale).
Deep cleaning is a manual abrasive cleaning process that ensures scale in toilet and urinal pipes are removed, preventing eventual blockage and flooding from occurring.
Such an on-going cleaning regime is ideal in new buildings where the slate is clean. All is not lost in old buildings! The situation can only be improved and kept from getting worse by implementing a frequent deep cleaning regime.