An advantage of building it internally is that the solution will be unique to your organization, and tailored to it. The organization will have complete control over the solution, including full ownership of the source code. This will allow the organization to modify it in any way that it wants. If a need arises to make changes or add functionality, the organization will be able to do so with full authority.
Another benefit of building it yourself is that there could be a competitive advantage. If the solution provides some feature that competitors do not currently have, building it and owning it could provide a strategic advantage to the organization.
The disadvantages of building it yourself are that it will require time and resources to do so. The end result may not have as robust a set of features as an externally developed solution. For example, if you are in need of a distributed, scalable, enterprise-level, full-text search engine as part of your application, it is probably impractical to build it yourself rather than use a proven solution that already exists.