1. Psychological Support
Using a demo gives you the opportunity to get a hold of handling wins and losses, of course trading with your own money is quite different in terms of the degree of strain it can have on you psychologically however using a demo can still provide a good way to practice losing which will help you in the long run because as a trader you have to get used to losing. Demo trading can also give you the confidence you need to successfully trade a live account, nothing is better than being profitable on a demo and taking that confidence over to a live account and being profitable there as well.
2. Familiarity with your Trading platform
Using a demo account doesn't only make you more familiar with the processes of trading it makes you more familiar with going through those processes on your chosen platform, you want to be able to know your platform like the back of your hand and placing trades and navigating through your platform should just flow with ease, you can only achieve this by spending a good amount of time (4-6 months) practicing
3. Build your Skills
Having a demo account gives you the perfect opportunity to try out new strategies and methods without having to risk any money at all, it gives you the chance to practice and practice until you feel you have the right tools and skills to be profitable on a live account.