One legit use is for companies/organizations that have a PBX (Private Branch Exchange) where each desk has it's own external telno. They can make all the calls appear to come from the corporate main number. So instead of 415-555-9xxx for whoever's extension it shows up as "415-555-9000". This is useful for both not revealing a personal extension and for branding

Of course, why it was designed to allow spoofing of any number in the world, instead of only numbers in the same block...