Why waste your money for something you don't need? The water conditioner you "need" [and the one that answers your initial question] is the one that will handle the issues in your tap water. Not all tap water is the same.
Prime is a very good water conditioner but it fiddles with all sorts of water chemistry, and that means adding chemicals to the tank and messing with the natural biology. If your tap water contains ammonia, nitrite and nitrate, fine; but if it doesn't, I would suggest a different water conditioner. There are many that do the job adequately, often for less money too.
There is another issue, and that is the live plants. These are nature's water conditioner, to a point; it is preferable to let them do their job. Some of the stuff that Prime fiddles with are essential nutrients for plants.
If your water only contains chlorine, you have a wide range of available conditioners. I am in this group, so I look for a water conditioner that handles chlorine and preferably nothing else. There are a couple. If you have chloramine as well, then you need a conditioner that handles that. Most (with a couple exceptions) will deal with chlorine and chloramine as basics. After that, the next thing they handle are heavy metals. Then some also detoxify ammonia. Then there are a few that also detoxify nitrite, and lastly nitrate. Unless you have ammonia, nitrite or nitrate in the tap water, you do not need a conditioner that deals with these. The plants need nitrogen, and they are very quick to grab ammonia/ammonium. Nitrite and nitrate will never be an issue in a planted tank that is not over stocked with fish.