Here's the good thing about margin accounts - they let you borrow money to invest.
Note that you can only borrow money using margin accounts. You can't borrow on margin using TFSAs and RRSPs.
How To Borrow On Margin
Margin accounts are fully taxable, which is why I recommend filling up your TFSAs first, but for young professionals, the ability to borrow gives the margin account another leg up on RRSPs.
How much can you borrow? That depends on a formula that I don't want to get into right now. There's a quick way to check, though ...