When the game that you are playing is fun, you really shouldn't need to buy into any of the gimmicks that the developers add in to make a little bit of money. The only reason you would need to spend money on a game that is free is if the thing that you are purchasing will help you win. Other than that, there is no reason to spend money on something that you got for free.
However, with Fortnite, you want to spend money so that you can get a new skin. There is no benefit to owning a skin on Fortnite. The only thing you get to claim is bragging rights when you own a skin that was available for only a limited amount of time. But those bragging rights make it very much worth the money that you spent in order to buy the skin so you can play with it.
Those bragging rights are not the only reason why you should get some new skins in Fortnite, though. There are plenty more reasons that make this a very validated purchase.
For example: You actually get more interested in the game if you have a character that looks similar to yourself,
If you have a friend who wants to play the game but can't seem to find a way to get as invested into it as you, you can get them a Fortnite skin that looks similar to them, or a skin that is from a certain comic that they are interested in, and they will be more interested in playing the game because they will have something that they are familiar with in the game.