There are three questions you should ask instead of one:
- Is there any reason for a borrower to borrow stocks other than betting that a stock is going to go down? The answer to that is basically "no". That's the main reason to do it. There are a few weird edge cases which are the exception to this general rule.
- Is there any reason for a lender to loan out stock? If I run a mutual fund, most of my stock is just sitting in a big dusty pile most of the time, doing nothing. I give the impression to my customers that I'm constantly buying and selling, but really, I just buy and hold most of the time. My main problem as a mutual fund manager is not the market going up or down. All of my peers are in the same market. My main problem is figuring out how to get a tiny edge over the other mutual fund managers, so my fund is on the top of the list at the end of the year. If I lend out my stock, my mutual fund might make a quarter of a percent per year extra. That's usually enough to bring my mutual fund up to the top of the list, relative to my peers. Profit.
- What is the benefit to the market of stock lending? As others have pointed out, the exact language you use to describe this transaction can be interpreted in a negative way. My opinion is that markets that allow this type of bet are much more resilient when the market inevitably turns down. Markets that don't allow stock lending tend to have artificially high prices for a long period of time, and inevitably crash hard. So, I think stock lending is a good thing. It evens out the highs and lows better.