The in-game currencies are a way for developers to earn money for game development and further improvements. Even though you often need real money to get them, these purchases are never pointless. In Fortnite, this currency is V-Bucks, for which players can buy a huge number of upgrades for their characters, which are mainly skins, accessories and skins for weapons.
If you would like to customize your character but don't have the savings to buy V-Bucks, you can get them completely for free. Just exchange your Soul Gems for 5,000 V-Bucks and you will no doubt get everything you need!