I think people who doubt the purpose and function of using bitcoin are those who often panic when prices fall. because they don't have any grip and belief in bitcoin. whereas the main advantages of bitcoin are clear, that anonomity  and decentralization are the main attractions of bitcoin, and you can say that fluctuations make bitcoin unsuitable for currency, but the fact is that many merchants and users use bitcoin for it. Bitcoin is needed for many reasons, and including what I mentioned.