Amenities can vary from nothing (if not available on a specific sailing) to onboard spending credits, bottle of wine, fruit baskets, cheese plates, photo coupons, photo albums, logo towels, robes, slippers, etc.
Typically the biggest advantages are the amenities, and sometimes discounted rates. RCI and Celebrity don't offer discounts on groups, but Carnival and some others do offer discounted rates on groups. Also groups can often secure the room categories (and even specific room numbers) that they want, keeping the group together if so desired.
And of course, there's always the FUN of cruising together with fellow Freeks!!!