Fidelity Investments is ramping up a business that helps money managers profit on those eager to bet against stocks.

Fidelity said this week it is launching a platform for fund managers looking to lend out their holdings to other investors, including short-sellers. Fidelity’s move is the latest sign that the money-management industry, which once dismissed the business as unnecessary and even a bit unseemly, has come full circle. Lending out securities has emerged as an important source of extra revenue, juicing returns and keeping clients from fleeing to cheaper investments.