Well, besides the reason I mentioned above, my guess is that people borrowing on firefish are hoping that bitcoin will grow faster than their interest and they will be able to recycle the same, or less, bitcoins in new loans. Firefish doesn't want gradual repayments, but whole amount plus interest at the end date of the loan. Similar to bonds.
So you can take the same amount of bitcoins and pay ahe old loan with a new one.