Git submodules are a powerful tool for managing dependencies in codebases, allowing developers to include external Git repositories within their own repository, making it easy to keep track of and update those dependencies. They offer benefits such as easily managing dependencies, branching out, and working on submodules independently of the main repository, but also have drawbacks like requiring separate tracking of changes, being tricky to work with if not familiar with the command line, and potentially increasing the size of the repository. Compared to monorepos and package distribution systems, Git submodules offer a cleaner commit history, better security, and are more suitable for apps using multiple languages or tools that don't support monorepo or package distribution. They can be used in CI/CD to manage dependencies, automate cumbersome merge processes, and streamline workflows, but should be used with caution and with an understanding of their drawbacks.