It is illogical, and that is why not many people know it or even do it.
But when you for example trade a future contract with an overnight margin of $2500 dollar, you can open a new contract when ever your account grows by $2500 dollar. So, if you know what you are doing, you could build up positions with market money. The big problem is margin calls, if your positions goes against you. If your account drops below the margin requirments, your broker will liquidate your positions and you hold the bag for any losses that comes with it.