Demo account is not useless, as the name denote and according to dictionary, demo means a demonstration of a product or technique. Although, in trading, it is better to practice with small amount of money like $10 to $15 with a less volatile market.
Demo is to test how trading works. After you check that, you can use small money to practice, while increasing it after some months if the profit is getting bigger and much more than the loss.
I remember when I saw an IQ option trading online, I downloaded an app for it, tried the demo and I lost all the whole fake money given me and I did not bother to use money to trade at that time. In crypto, no even better deme when I started, I just use video guides that the exchanges provided and I used small amount of money instead of demo, but I can easily first just start with demo and use it for like 30 minutes to see how the trading works and move to use small amount of money.
I too do not see demo to be necessary, but not useless to be seen to demonstrate how trading works, but I wonder why people turn to it as a means to help during trading, which is not, small amount of money should be used for that, not demo, but demo can be used for testing how trading works.