Why pay the overhead of a large visual effects facility when all you need is simple, invisible effects?
You can hire one person (or a few people) to work with your editors and directors to fix little things quickly for a fraction of the cost of an outside vendor.
Temporary visual effects provide editors and directors footage that they can cut and screen a movie with.
Know that they're gonna get the movie they want before they spend the money on final visual effects.
Sometimes you just need someone to track shots for you. This can happen at your facility or offsite.