borax is used for two distinct things:
1) to raise pH without having any great impact on the total alkalinity.
2) in much larger quantities to bring the borate level to 50 ppm to act as an algaestat and to introduce a secondary pH buffer that works in the opposite directon of the bicarbonate buffer we call TA. This tends to 'lock' the pH at around 7.7 for an extended period of time and is particularly useful with SWCG's (salt pools) but useful in any pool, IMHO.It also gives the water a clarity and 'sparkle' that has been commented on time and again. Much larger quantities of borax are needed to get to 50 ppm borate and since borax is alkaline, unless acid is added in the correct quantity at the same time to keep the pH in check, the pH would be extremely high.
As to why 50 ppm--the original research and patents by John Garvin, Proteam, and Bioguard determined that 30 to 50 ppm is optimum for algaestatic and buffer effect in chlorine and bromine systems and that 50 to 80 ppm is needed for algaestatic effect in biguanide (Baqua and SoftSwim) systems.