I’m going to answer your questions from my experience teaching git and GitHub as Campus Expert.
What is the purpose of the Pull request and merge request?
This is a way to collaborate in software projects though the version control (aka git), GitHub have the PR Pull Request, and other platforms implement the same (GitLab, BitBucket, etc) with different name.
Why should we make use of these techniques?
One of the main goals for git was to be distributed, this technique is very useful, if you want to see the power of this, maybe you’d explore more open source projects and see how this projects are collaborating with a lot of people using the PR for collaborate.
The Spring Framework repository has a great contribution guide about how to make a PR for contribute to this framework repository, this is a great example about why you should use and learn this techniques.
Can these techniques be used for the .Net project?
Remember: git it’s like a mini file system, it works with any kind of file, so, it doesn’t matter if you are working with a .Net project, with .txt, with java, html, markdown.
If it can, how can it be used?
Although there are a lot of books and courses my recommendations is learn using the GitHub Learning Lab, and reading the oficial documentation of git.
As a single developer where I create an individual application, how does pull and merge request would be a benefit to me?
Maybe the PR don’t make sense for individual private projects, in this case you should know more about git workflows. The PR is from GitHub workflow, but there are more workflows for version control systems: git flow, feature branch, tree based development, etc… every workflow could be implemented for collaborate alone or with a team.
Greetings from Mexico!