A pull request (PR) in Git is a feature that facilitates collaboration and code review in distributed version control systems, particularly when using platforms like GitHub, GitLab, Bitbucket, and others.

It is also sometimes referred to as a "merge request" in certain version control systems.

A pull request is typically used in the following scenarios:

1. Proposing Changes: When a developer or team wants to introduce changes (new features, bug fixes, improvements, etc.) to a Git repository, they create a branch in their local repository to work on those changes. After making the desired changes in this branch, they create a pull request to propose merging their changes into another branch, usually the main branch (e.g., `master` or `main`).

2. Code Review: The pull request serves as a way to initiate a code review process. At this point, any team members or collaborators can review your code, leave comments, ask questions, and suggest modifications. The review process helps and ensures that the proposed changes are of high quality, adhere to coding standards, and it does not have any issues.

3. Discussion and Feedback: The raised Pull requests provide a platform for discussion and feedback related to the changes being proposed. You can have conversations within the PR's comments section to address concerns, provide clarifications, and refine the changes.

4. Continuous Integration (CI) and Testing: Many Git hosting platforms integrate with CI/CD (Continuous Integration/Continuous Deployment) systems. When a pull request is created or updated, automated tests and checks can be triggered to ensure that the proposed changes do not introduce regressions or break existing functionality.

5. Merging Changes: Once the code review process is complete, and any necessary changes have been made and approved, the changes can be merged into the target branch. This can be done by the person who created the pull request or by another authorized team member with merge permissions.

6. Tracking and Documentation: The raised Pull requests provide a structured way to track and document changes made to the codebase over time. Those include information about the proposed changes, who made them, when they were made, and any associated discussions.

The key steps involved in a typical pull request workflow are as follows:

1. Create a feature or bugfix branch based on the target branch.

2. Make changes in the branch and commit them.

3. Push the branch to the remote repository.

4. Create a pull request, specifying the source (feature/bugfix branch) and the target (usually the main branch).

5. Reviewers review the code, and discussions may take place.

6. Address any feedback and make necessary changes to the code.

7. Once the pull request is approved, it can be merged into the target branch.

Pull requests are very important in software development and especially for open-source projects when multiple team collaborates.

They help in maintaining code quality, transparency, and communication among team members.