What is the use of fermentation in pickled vegetables?

Fermented foods have an extended shelf life. Pickling was done traditionally to keep excess crops from spoiling and to preserve them during the long winter months.
Pickles, sauerkraut, even certain deli meats could be prepared with a brine solution. This process slows down bacterial action that can lead to spoilage.
Vinegar can be used to help preserve foods because it prevents the growth of bacteria that cause food spoilage. Salt brine allows the process of fermentation by bacteria which is used in producing pickles.
Fermented foods can have health benefits. Lactic acid bacteria use the carbohydrates in the food to obtain energy and through a metabolic pathway, produce lactic acid. This gives the sharp taste associated with fermented foods. However, additional health benefits are produced when enzymes and vitamins are produced as well.
Worldwide, most cultures have used fermentation and pickling as a way to extend the shelf life of the precious food supply.
https://www.exploratorium.edu/cooking/pickles/pickling.html