The bigger and more established the company, the more work they have to do for all their different markets. If you look at Coke for example, they have to create collateral for consumers around the world. They very likely have a fantastic in-house team but their team doesn't speak every language, doesn't have time to create a set a digital/print ads for thousands of different advertising markets. While they might come up with design layouts and ideas, they outsource the rest because its often more effecient. They might just outsource busy tedious work, like resizing ads for different markets or they might outsource a job to a big production company because a production company is going to have the tools and talent to create a commercial or mobile app. Often times you have due dates and if the in-house team is too slammed to get everything done, they might outsource work as well. For example, a new campaign is supposed to launch in a week, and they need extra help from a company they trust who knows their brand.