Regardless of the category you select (and the amount you pay), you're on the same ship going to the same destinations; you dine in the same dining rooms; you enjoy the same activities and entertainment; and all of the same interior and exterior public areas are available to you.
It's not like the movie Titanic, where the First Class passengers were pampered in total luxury, while the steerage passengers were little more than human cargo.
So, the difference is the staterooms themselves. Some people wouldn't dream of crusing without the convenience and privacy of their own verandah. Others would rather the save the significant cost of a private verandah, and use the wonderful public outdoor spaces on various decks instead.
Also, stateroom capacity is an issue. For example, the cheapest verandah staterooms (Category 7 on decks 5, 6, and 7) only hold 3 passengers, so they're not available to families of 4.