Why use GitHub Desktop?
It is true that the Git CLI tool is powerful, and you will be just fine if you stick to it. However, we, full-stack developers, are worried about a variety of concerns. In your rush to complete the task at hand, you can easily ruin your and sometimes your team's day, by following bad or incomplete advice.
See the following screenshot of such advice from StackOverflow (http://stackoverflow.com/questions/1125968/force-git-to-overwrite-local-files-on-pull):
If you execute the preceding command, be prepared to lose uncommitted local changes. Unfortunately, novice users have a tendency to follow the simplest and most direct instructions, potentially leading to lost work. If you think your past commits are safe, think twice! When it comes to Git, if you can imagine it, it can be done through the CLI.
Thankfully, with GitHub, you can protect branches and implement the GitHub workflow, which entails branching, committing, merging, updating, and submitting pull requests. The protections and the workflow help prevent harmful Git commands from making irreversible changes and enable a level of quality control so that your team remains productive. Performing all of these actions through the CLI, especially when there are merge conflicts, can get complicated and tedious.
For a more in-depth understanding of the benefits and pitfalls of Git and GitHub, you can read my 2016 article on the topic at: Bit.ly/InDepthGitHub.