I do the Hopper Plus often. If you add it onto your ticket you get additional days equivalant to the number days of your parks days ie.if you have a 5 day park hopper, you get 5 “extras”. Those extras can be used on days outside your park days at Blizzard Beach, Typhoon Lagoon, Fantasia Gardens Mini-Golf, WinterSummerland (or whatever its called) Mini-Golf, ESPN, and a round of golf at the 9-hole Oaktrail. Back in the day you could use it at Disney Quest.
It gives you more options in planning your days, and you can buy a shorter number of ticket days. For example, we have a short trip coming up in July and we’re only buying a two-day park hopper plus. Two days will be spent in parks (we can visit one during the morning and a different park at night), and two other days at the water park/golfing- and doing other things like Disney Springs and being in our pool. That’s an example of how we use it.