1. Dependency Injection: .NET Core encourages the use of dependency injection to achieve loose coupling and improve the maintainability of your application. By registering component types in `Startup.cs`, you inform the DI container about the dependencies required by your application's services and components.

2. Service Resolution: The DI container needs to know how to resolve and provide instances of the registered component types when they are required throughout the application. By registering the types, the container can resolve these dependencies and inject them into the appropriate locations, such as constructors or properties.

3. Lifetime Management: When registering component types, you can specify their lifetimes, such as transient, scoped, or singleton. This controls how instances of the component are created and managed by the DI container. Registering the types in `Startup.cs` allows you to define the desired lifetimes, ensuring proper management and reuse of instances throughout the application's lifecycle.

4. Application Configuration: The `Startup.cs` class is the central location for configuring various aspects of your application, including services and middleware. Registering component types within this class helps keep the configuration organized and centralized, making it easier to understand and maintain the application's dependencies.

5. Testability and Modularity: Properly registering component types enables easier unit testing by allowing you to substitute dependencies with mock or fake implementations. It also promotes modularity and separation of concerns, as you can swap out implementations without modifying the consuming code.

By registering component types in the `Startup.cs` class, you ensure that the DI container is aware of the application's dependencies, can resolve them correctly, and manage their lifetimes effectively. This promotes maintainability, testability, and modularity in your .NET Core application.