Films, TV shows, and commercials aren’t done once the shooting stops. They might have all the footage they need but they still need to edit the videos to get the right sequence and cut anything unnecessary. They might have to improve the look of the shots through color grading. They may also need to add visual effects to make scenes more engaging.
VFX is a highly specialized part of post-production. Only a few experts have the equipment, skills, and experience to pull them off in a realistic manner. That’s why most companies rely on others to do it for them. That is what is VFX outsourcing: letting another company handle this matter so that the can focus on their main tasks.
This makes more financial sense since VFX may not always be needed in films. The specialists can come in when needed. It is not necessary to maintain an in-house team.