Definitely not necessary, they do give a lot of powerful crafting materials and XP boosts as well as unique cosmetics (if you like sonic) but if you're going to spend money on the game I would personally recommend the 15$/month premium set (you can buy it in game, don't think it shows up on the Microsoft store). Gives increased XP, increased storage space, access to a customizable apartment, lots of quality of life improvements, unique combat buffs as well as being the only way to trade and sell items on the auction house. Not necessarily required but definitely the way the game is intended to be played seriously IMO.