i'll give you the real answer op since you made an epic thread.  it's not the pickle, it's the pickle juice.  it adds a bit of salty water to bring out the other flavors.  that being said, there are many burgers that don't come with pickles like my 2 favorites: the shroom and swiss and the patty melt.  although there is likely a ton of jewishness at play here, i believe that pickles are a classic sandwich ingredient and a cheap way to add an extra level of flavor
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