When get big in your trading which is measured in 1 BTC at their exchange , you're required to send them some document for you to be able withdraw >1 BTC. Good thing that those popular exchange still gives chance for some newbies to learn the ropes in trading without risking their identity and small fiat moneys that much first. When I planned to start day trading, I thought that I wouldn't be able to do it easily since some exchanges require me to pass this KYC. Good thing that it doesn't require you and your identity just to at least try and learn the trading basics.