apparently hops is the bittering/perservation agent in beer.
below is an excerpt from http://www.brewersconnection.com/popup/whatarehops.htm

But why are Hops in Beer?
It has been long speculated that hops were first introduced into beer for it's preservative properties. The lupulin in the hops serves as a preservative. They add stability to beer due to their antibacterial properties. The flavoring and aromatic agents were an added bonus. A bonus that has caused this wonderful and mysterious plant to evolve into what it is today, and will only be better in the future. Hybrids are continually being developed known as "dual purpose " hops.
The Hop cone contains 3 different resins, alpha, beta, and gamma resins. The alpha and beta are often called the "soft" resins and the gamma resin is "hard" and contributes nothing to the brewing. The "soft" resins add the flavoring and aromatic agents. The alpha resin is intensely bitter and becomes soluble as alpha acids in the boiling wort. The alpha acids are isomerised during boiling to the even more bitter iso-alpha-acids. Hence the long boil of "Bittering" hops during a hop schedule. The isomerization of alpha acids add bitter-sweet balance to the finished product. The iso-alpha-acids also supply tannins that combine with unwanted proteins. Tannins are complex, generally oxidized poly phenol polymers. When proteins and protease come into contact with these astringent-tasting flakes, they adhere to them and by virtue of their increased mass are precipitated out of solution. This helps to precipitate out unwanted protein matter that can haze or cloud beer.
In contrast, the beta resin group or oils are far less stable than the alpha-acids. They are extremely subject to oxidation and are only slightly soluble. They are very volatile and rapidly diminish or evaporate during the boiling process. However, these resins are responsible for most of the "hop character" of a beer, i.e. hop flavor and aroma. Hence the late kettle additions of a hop schedule. These essential oils have eluded and mystified the beer comunity for eons. In hops, the essential oil makes up only about 0.5-3% (vol/wt) of the whole cone, but the contribution it gives to beer is huge. One can deduce their presence or absence by simply using our noses. You can capture these oils for it's aromatic value in many different ways. One popular way is to run the hot wort through a bed of hops before going into the chiller. For the typical home brewer, this can be hard, if not impractal to do. One easy way would be to save off a small amount of wort, about 2 cups, and after you wort is cooled and in the fermenter, heat the wort you saved off to about 180 degress, turn off heat. Add hops and let steep for 5 - 10 minutes, and then strain and add this mixture back into your wort. It is important to be sanitized during this process however. Another method used by some home brewers is to make a "Hop Tea" in the same fashion as described above and add it 4 - 5 days into the fermentation process. And then there is what is known as " Dry hopping" or adding hops to the fermenter is another method that capture these oils. But be advised, the compounds extracted by cool wort or beer can be quite different than those found in a wort after the boil.

hope this helps

best regards

I have some great information on hops here hope this helps to better understand why hops are used .

http://en.wikipedia.org/wiki/Beer_Hops

Bitter herbs were often used by early beermakers to offset the sweetness produced by the fermentation process. The hop plant is native to Europe, and by the 11th Century it too was being used to make beer. Unlike other bitter herbs, hops have antibacterial properties and serve as a natural preservative for beer. The use of hops was codified by the German Purity Law of 1516, which stated that "the only ingredients used for the brewing of beer must be barley, hops, yeast and water."