In this way the broker makes trader to fund his account and start trading with real money. Of course, it would be better to have enough time to practice - each trader should decide himself when it is suitable for him to start trading with real account. At the same time, since it is quite easy to open demo account and since it creates no obligations for the trader, it is possible to open multiple demo accounts one by one to be able to get strong tradin skills before starting trading with real money.
Some of the traders also use demo accounts to test their strategies even if they already have real accounts just because it is better to check the performance of new stategy using demo accont rather than to risk with real money. At the same time, this is not the most efficient way of testing and its results sometimes are not accurate enough. If you are going to test the same strategy with different parameters on demo, you wouldn`t be able to get detailed results just because the data and the number of trades would be different for each time. The results of demo testing could be also influenced by trader`s psychology. That is why to my mind it is better to use automated backtesting with the help of scripts (for those, who have deep knowledge in coding) or with special software like Forex Tester which would be suitable even for novice traders.
At the same time, demo testing is important too, because trader has to check his own performance with the new strategy. It should be comfortable for him to trade using it otherwise he would more likely to violate its rules. So the optimal strategy development process includes both automated backtesting and manual demo testing.
That is why demo account is still an important tool for traders
Demo test is often sufficient approximation of performance on the live account, moreover you don't lose anything from conducting tests on demo (except time), so I advice to perform tests on demo first regardless of potential performance discrepancy with live account. Demo testing results is new information which is likely to be helpful.