Why do I need to provide my financial information when opening a forex account?

Well, I don't know about all trading platforms. Lite forex for instance could ask for your bank details so that they recognize that it's you when you're depositing from your bank account into your forex account, and also so they they can know which account to send the money to in case you want to withdraw from your forex account. This is my assumption though. You should always be careful and vigilant when it comes to giving out your financial details.