Not necessary. Formula is just a manmade liquid diet for infants and meant to be a complete food. It doesn't taste particularly good, but it is complete nutrition for an infant. For a small infant, it will be the only food s/he is taking. For an older infant, closer to 12 months and eating food, formula may not be necessary.
As supplement. Eventually, most parent's will supplement breastmilk with formula or go directly to formula. Other times formula is needed due to food allergies. There is no good or bad reason to give or not to give formula. Getting used to the taste will not be a problem although different types of formula, regular versus extensively hydrolyzed, can be terribly different tasting. To find out, try them yourself.
No. You do not have to give tastes of formula to a baby for an eventual change from breast milk. If you are weaning from the breast, this will be the time to gradually change over to formula. You can even just change over. There may be protesting but babies are absolutely incredible at making the change will continued feeds of formula. Hunger takes over and they just do it.