Here's what wiki says about it.
Sometimes used in the brewing industry to inhibit the growth of wild yeasts, bacteria, and fungi. This is called 'stabilizing'. It is used both by homebrewers and commercial brewers alike. It is not used as much for brewing beer, because the wort is almost always boiled, which kills most microorganisms anyway.
'Sodium Met' is probably the most familiar 'no-rinse' sanitiser to homebrewers. It is sold as a white powder in all homebrew shops and most supermarkets that carry a range of brewing ingredients, and it is found in most starter kits. Unfortunately, it is one of the worst sanitisers available now that so many better alternatives exist.
Sodium Met is slow to act and often used ineffectively. To use it properly, dissolve the sodium metabisulfite in the correct amount of water, as directed by the instructions, rinse all surfaces with the solution and allow to air dry. Drying is essential because this is when the sodium metabisulfite does its thing. It does not kill bacteria outright but it inhibits the growth of wild yeast. The fumes are highly irritating (particularly to asthmatics) and should not be inhaled.
It does have other uses around the brewery though. Sodium Met (as powder or in the form of Campden tablets) is sometimes added to wine or cider to halt the fermentation process before it is complete, to leave some residual sweetness in the must. It is also effective in purging chlorine AND the more stubborn chloramines from brewing water, and some brewers add a pinch or two to the mash, in the belief that it protects against certain undesirable reactions and helps to preserve the finished beer. It is also useful as a way of sanitising water to use for rinsing other cleaning or sanitising products. "
I use potassium metabisulfite in wine to inhibit wild yeast growth, some beer brewers use sodium metabisulfite for sanitizing but for the home brewer there are a lot of cheaper (and safer to the brewer) sanitizers, iodophor, starsan etc.