"...We want a class of good bacteria, called lactic acid bacteria, to colonize the cucumber and start to preserve it, before the bad bacteria invades and starts to rot it. Both sides use a form of natural chemical warfare, starting with fermentation. In fermentation, bacteria of all kinds, good and bad, eat and digest starches like the natural ones in cucumbers and other plants and convert them into sugars they use as food and fuel to grow and multiply. As soon as a cucumber is put into salty brine, the good microbe army has the edge."
Basically, the brine -- remember, it's got the acidic vinegar -- becomes an idea medium for the good microbes to ferment the pickles without causing them to rot. It also increases the acidity of the overall solution and gives pickles their distinct, sour taste, while simultaneously preserving the pickles for quite a while. Per Mr. Roker:
"...By growing rapidly and producing all that lactic acid, the good bacteria increases the acidity of the brine and what was soaking in it, which decreases the pH to below 4.6, a level acidic enough to pickle our pickle, and keep it preserved for months, even years."