The profit from trading is based on a small percentage from the sale, not the full amount. For example:
You buy 1 btc for $500, then sell it for $505. This full $505 amount is not all profit, because you bought it for $500. You really only made $5 on the trade, or 1%. You're doing large transactions for small profit, which is the margin.
This is not a 100% true statement. How margin works is that you only pay for some percentage of the full price of the investment. Say for example that BTC is trading at $500 but you only have $200. You could borrow $300, add it to the $200 that you have and buy 1 BTC. If the price were to go up to $510 then you would have a profit of $10 minus your interest costs.
If you were not trading on margin then you would have only purchased .4 BTC for $200 and when the price went to $510, your position would only be worth $204. So your profit (and potential losses) are magnified.