You certainly don't have to boil the vinegar or brine when pickling, but doing so offers tasty benefits. First, a warm pickling liquid will permeate the vegetables better. Especially if you also briefly boil the vegetables before combining. Warm ingredients speed up the pickling process, and you'll be enjoying tangy, sweet, crispy goodness sooner. Second, boiling the brine ingredients allows the salt and sugar to dissolve nicely into the liquids. It also releases volatile oils from any herbs and spices. And this creates deeper flavors that would otherwise remain untapped.