Have you been invited to one?
If so, they obviously like to have parties, because engagement parties are not at all necessary. It's often a cocktail party sort of affair.
They are NOT a gift giving occasion, but if you have reason to want to bring a gift, the best (and only one) I would suggest would be a copy of Emily Post's Etiquette, or Emily Post's Wedding Etiquette- but that would be if you were a close friend. Otherwise, nothing.
It's really an excuse to have a party to celebrate the couple's recent agreement to marry. No one should be expecting to be named bridesmaid, Best Man, or anything else- that would be premature.
If done right, the "engagement party" would be a small dinner involving both families or even just the parents, to allow them to meet. But these days, too many people are addicted to Pinterest and use every excuse to have a do. That's all. Nothing special happens.