Remember how in 2008 the entire world was about to grind to a halt because credit markets were freezing up? This is why. It's the norm because mismatches between when bills are due and the money to pay them comes in are common. It's similar in function to payday lending, but the terms are better, and it's not done under duress. Businesses routinely take out loans to make payroll while they wait for all the money they made from the work done, for example.