Perfect Fit sleeves are primarily used for collectors or even players looking to add an extra layer of protection to their cards. Their tight fit is excellent for keeping out moisture and dust, which standard sleeves cannot do to nearly the same effect. The only major disadvantage is that they do not protect against scratches as well.
They also enable one to "double sleeve" by applying a perfect fit-sleeve to a card with the opening at the bottom of the card, and then a regular sleeve with the opening at the top of the card. This practice offers yet better protection against dust and moisture, and can even keep out fairly large volumes of fluids (say, for example, you're at league and someone spills their drink on your deck. Double-sleeving may save your expensive Virizions and Genesects).
Generally speaking, unless you're a serious player or collector, it's not necessary to use perfect-fit sleeves. However, if you would like to provide your cards with the greatest protection, they are part of the solution.
Also, don't put any somewhat valuable cards back-to-back in sleeves. That comes with a large risk of scratching them, even if it's just the back of them.