I often have been asked that why we need pull request.
If your workflow have pull request in it then it will help you to start a code changes and discuss the solution with your team members and eventually come up with a new feature in your application. It helps you to discuss code changes asynchronously with other person.
Particularly in the world of open source, pull request really shines. Any one can contribute in your codebase and they can be outside of your team, may be they are in a different time zone. In this case it is easy to open a pull request with a code change and work asynchronously with other members. GitHub has a very sophisticated code review process around pull request.
Overall it is the new way of workflow in a software project where you can collaborate with your team members or anyone who wants to contribute. Also you can propose code changes to other’s repositories and the owner of the repository can review your changes before merge through pull request.
If you attach issues with pull requests in GitHub then after marge in the future you can check the issue and able to find all the code changes related to that issue in the attached pull request.