You want to make payment in BTC now, but must acquire BTC first. Solution: you lend BTC now, and acquire BTC right thereafter in exchange for dollars afterwards to pay back the loan. Could still be cheaper than regular payment if the fees are stiff.
correctIon: you *borrow* BTC now ...
Yup, the delays in getting funds moved to the exchanges are a big reason. Sending funds to an exchange can be done using cash deposit at a bank nowadays using BitInstant, for example, but that is in the 4% fee range and takes a trip to the bank. If a BTC line of credit could be drawn from, then the purchase using the proceeds of the loan can occur today. Then when the cash transfer via electronic means (e.g., through Dwolla, for instance) occurs bitcoins can be bought later and the loan paid off.
Where I live (USA), there is not much I can think of off the top of my head that you would need to borrow (say) 20BTC for and then actually be able to pay back 22 (or whatever) in a few weeks.
There are lot of instances where BTC funds are the only or best payment method. When the seller dictates the payment method the buyer complies. And there are other reasons as well. Like to pay off a previous loan. (Ya, taking a loan to pay off a previous loan -- crazy! Who do they think they are, the U.S. Government?) And, don't forget, some of these requests are being made by scammers -- either with the intention of defaulting when taking the loan, or with the intention of repaying and building a good reputation history (see definition of "confidence game") to be able to do a bigger scam down the road.