I think demo accounts are great for having a proper learning experience and practice. A newbie can’t always handle the pressure of a live account and I see no harm with demos where they don’t have to worry about losing their money.
For me, demo accounts have proven to be a great medium to practice. They have helped me in getting to know about the trading conditions without any risk of actual money and I consider it one of the best sources of learning especially for beginners.
Yes, it’s more than just necessary. It's important. You tend to increase the risk of losses if you directly enter into the live markets. So invest some time in learning through a demo account and then after you’ve learned enough, shift to trading markets.
Demo account is not necessary but it can help you understand the trading market in a better way, that too without risking your money. In my initial days, I demo traded for 3 months just to be confident enough before going ahead with the live accounts. I learned almost everything from trading to spreads and how actually the indicators are used. These things help you in your future.