The reason why you boil vinegar when pickling is to help dissolve the salt and the other spices in the brine.
Boiling the vinegar when pickling also activates the spices flavors and achieves a more even distribution throughout the pickling liquid.
Essentially boiling the vinegar creates a hot brine that can better penetrate the vegetables and also ensure the proper preservation by killing any unwanted bacteria which may be present.
The high heat from boiling the vinegar also helps eliminate any harmful microorganisms which can spoil the pickled foods.
And boiling the vinegar allows the flavors of the spices and the herbs to fully meld together into the vinegar.
And depending on the food or vegetables being pickled, boiling can also soften the texture and it can be desirable for certain pickles and pickled foods.
Some pickling methods, such as for delicate vegetables will use a cold brine where the vinegar is not boiled.
Most quick pickling recipes involve the boiling of the vinegar mixture in order to achieve a fast pickling process.