The rise of the “gig economy” has prompted much soul-searching about the future of the economy. But portfolio careers aren’t only for stay-at-home parents looking to freelance a few hours a day while their kids are in school or grad students moonlighting as Uber drivers and Task Rabbits. Even senior executives who want to stay in their corporate jobs should strongly consider developing at least one side income stream, whether it’s consulting, speaking, coaching, or creating some other product or service. Here are five reasons it can make a dramatic difference for your career.