If a person is lucky enough to get a project or work with Disney, then that is a major boon for one's career. In fact, a person might be able to exclusively work for Disney their entire career, making a lot of money and success for themselves. Contracting with Disney is helpful to a career as well.
I believe that working for Disney can help a songwriter's career. Disney is one of the most recognized brands in the world. The company is also very respected for all of the company's productions. Millions of people will watch a specific Disney movie. For a songwriter, having one of your works in a Disney Movie can help you make a name for yourself.
If you want to get your songs heard by as many people as possible, especially those who will listen to them over and over, few places are better than Disney. Menken rose to fame as a composer for Disney films, and this is what earned him his huge collection of Oscars.
Disney is a Fortune 500 company (currently ranked at 53) and can no doubt be a valuable asset to any aspiring songwriter's career. As shown in the example, Alan Menken has won eight academy awards through working with Disney, jolting him to the Oscar's winners list and cementing him in history as a phenomenal composer and songwriter. I believe Disney can be a boon to anyone's songwriting career as the company gives them a prolific platform to get their ideas across.