So taking a swap doesnt give you actually the USD you loaned? Its only possible to use them for margin trading? If so then it would be a bit confusing. Giving loans you need actual real money, but taking loans only gives you something virtual.
Yes, swaps are explicitly
only there to provide leverage in margin trading. What else exactly were you planning to do with the funds?
It has to be real money because it's going to go into trades with other users who might want some actual money in exchange, but it's "virtual" to prevent the swap recipient being able to run away with it. Similarly (in the sense that it's also to prevent losses) they're limited to borrowing only a multiple of the value of their deposit, and will be liquidated when necessary to pay off the swap.
The whole setup is designed to, as far as possible, guarantee that no funds will be lost by lenders. Or rather, by Bitfinex, since they've supposedly extended their "insurance" option to all swaps by default.