Powerful software is often developed in-house. However, there are times when a new feature needs to be added to the system and it's simpler and faster to use a tool already developed by another company (or "third-party") rather than build that feature internally. This is an example of when you integrate instead of build from scratch.
Your business needs to respond to market demands quickly. Third-party integrations allow you to quickly add features and tools to your business processes that might take many months to develop on your own. Even if you have the resources to develop tools in-house, those tools might need a great deal of time to refine. Tools developed by specialists have already been through this refinement phase.