Sodium Met is the only legally allowed preservative in beer, its an antioxidant and is used by many in the mash to reduce the chance of HSA. 1/2 a tab can be added to 50 L of water to neutralize chlorine. It isnt effective on certain types of bacteria, it merely 'stuns' them allowing them to wake up later (in the middle of a freshly prepared wort dinner). It prevents the growth of wild yeast allowing brewers yeast to 'take hold' during fermentation, it can then be used (usualy in wine making) in a large enough quantity to halt the fermentation. Its also good for sanitising fruits and stuff before adding to the cask/secondary in much the same way as they prepare fruits for wine making, the difference is an unfermented wine must is a much less hospitable enviroment for bacteria than a wort.
Sodium met has a place in beer making but not as the primary means of sanitation.
BTW its particularly good at rinsing stuff that has been cleaned in bleach as it effectively neutralizes the chlorine and doesnt need to be rinsed.