Our friend COL Maxwell has a piece at Small Wars Journal on the subject of why the Special Forces train for unconventional warfare (UW). It's easy to understand the sensitivity of the question by reading the definition of UW:
"Activities conducted to enable a resistance movement or insurgency to coerce, disrupt or overthrow a government or occupying power by operating through or with an underground, auxiliary and guerrilla force in a denied area."
Why would we train for such a thing? The short answer: if you can carry out UW missions, you can do all the others. Read why.