Why does TypeORM need reflect-metadata?
With the reflect-metadata package you can do runtime reflection on types. Since TypeORM mostly works with decorators (like @Entity or @Column), this package is used to parse these decorators and use it for building sql queries.
The following link provides detailed information about what this package is capable of: http://blog.wolksoftware.com/decorators-metadata-reflection-in-typescript-from-novice-to-expert-part-4.