Backed by Japan’s SoftBank Group Corp., New York-based WeWork has built itself into one of the world’s most valuable startups through a global leasing spree. It had more than 300 locations in 83 cities as of the third quarter, and it says it’s the biggest private office user in Manhattan, London and Washington, D.C. In many ways, it has been a boon to the real estate industry by helping landlords fill spaces and generating big fees for brokers.