If an app enables the user to accomplish only one goal, then the user will be lost as soon as the one goal disappears or is accomplished better by using some other product. Accomplishing multiple goals makes for a sturdier foundation of usage that is more resilient to changing life circumstances, as User goals change over time.
Imagine if all LinkedIn was good for was finding a job. If this were the case, then people would stop using it as soon as they found a job. However, they can also be a good source of business news and updates about their colleagues.
Facebook also enables the user to accomplish multiple goals as a strategy for retention. Many of my friends have stopped using Facebook, but it's still valuable to me for the Facebook groups of local professional communities and for social event planning.