I'm heavy into a filtering right now, popping a trio of whites through a fairly tight diatomateous earth medium in anticipation of bottling all next week.
A lot of people ask me why we filter the wine, especially if they get a chance to see wine at some stage where it looks brilliant already.
There's a couple reasons that come to mind, I'm sure there are more if I thought about it a little longer.
My number one reason is: to the reduce the risk of microbial development in the packaged product that would seriously alter or ruin the character of the wine. Wine comes into contact with lots of bugs, many of which are microscopic. While the wine's acidity, alcohol and other factors make the environment inhospitable to many life forms, other species adapt quite well and flourish in wine conditions.
It would be rare to find a wine that would make you physically ill (except in cases of over-consumption, something I know very little about) but there are lots of situations where the wine becomes unfit for drinking due to microbial growth. Sulphur compounds are the dominant preservative used to make wine clean, but there are a host of other precautions used (like filtering) in concert to create a sound product.
Correctly managed, many wines can be bottled unfiltered. What the winery or the winemaker is saying by doing that is that the chemical composition of the wine is such that the possibility of the wine becoming flawed is highly unlikely. It's a testament to good wine making practices and meticulous cellar protocols.
Another reason for filtering: the public expects a brilliant product. Anything less than excellent clarity is considered a flaw even though the wine is sound from an organoleptic point of view.
White wines are almost all filtered. Many whites have residual sugars which provides fuel for many bugs. Sort of like an ammunition dump for terrorists.
On the other hand, reds are often unfiltered as they naturally suppress the chance of deterioration due to creature invasion.
Hope these brief notes help next time you lift a glass or peruse a label.