Wall St "repos". Legitimate financing or just plain gambling?

It's a very short term lending scheme.  The borrower sells some financial paper (often US T-bills) to raise money, but promises the buyer that they will buy the paper back shortly, at a small profit.  The deal is actually a short term loan, but on paper it looks like a sale. 
   Question:  what useful purpose is served by short term loans?  Legitimate reasons for borrowing are to build or buy plant and equipment, buy inventory for later sale, finance real estate (buying, building, whatever) finance research and development, in short to finance useful economic activity.  All of this stuff takes time, years sometimes, to pay off.  Short term loans, over night loans in some cases,  don't cut it.  All I can see a short term loan doing is making your books look better (more cash on hand) for a financial report, or to finance stock trading.  I think short term loans are gambling pure and simple and ought to be taxed out of existence. 
   Used to be, J.P. Morgan handled a lot of these deals.  For undisclosed reasons J.P Morgan decided to get out of the business last year.  Perhaps they see it as dangerous and high risk. At this point 85% of the deals go thru Bank of New York.  Wall St players are worrying that if anything goes wrong (power failure, fire, flood, hacker attack, whatever) at Bank of New York, the whole repo market is toast.  A good reason not to get into repos in my book.
   Repos are widely seen as one of the triggers for Great Depression 2.0.  Fear of repos caused everyone to stop lending to Bear Stearns and Lehman Brothers, which crashed both firms in the fall of 2007.  The Federal Reserve has wanted to overhaul the repos business for nearly a decade (with little luck).  The Treasury's Office of Financial Research estimates the repo market at $3.5 trillion, which is a lot of money.  Enough to touch off another great depression if things should go wrong.  
   Sounds like a land mine waiting for someone to step on it.