A demo account can give experience, but success in a demo account does not mean success also in the real trading account, there are so many factors that cause failure in real trading although get success in demo trading, one of them about psychology, trading server, slippage, etc.
this is a perfect answer.The demo account is basically a tool used to develop new skills and strategies without any real risk or capital involved.
When I moved from demo to live trading, I was not able to make consistent profits as much as I made in demo trading and the market felt more competitive.