I would like to share my reasons, based on what I learned from other developers, to have personal projects outside of professional jobs.
Unless you are a super genius, who can implement newly acquired knowledge flawlessly at once, it’s inevitable to make mistakes in programming based on new ideas. Your choice for solutions will need to be revised repeatedly. It may require number of customizations to accommodate merging these solution with codes from other developers or from older projects, though they are working just fine with no problems by themselves.
At early stage, your professional job will provide this kind of challenges many times, allowing you to grow patience in dealing with errors and testing implementations to find the right solutions. Sooner or later, you may find yourself simply customizing lines of codes for already solved problems. In other words, your job will get easier as your professional experience grows.
Unlike many other professional fields, new technologies are introduced so often, much faster than we can keep up with. But until your project manager or supervising developer decides to use these new technologies, you may only hear about them, if this is the worst case.
Your ability to implement new ideas, that you’ve practiced so hard, will become dull or inefficient. And what makes it more depressing is that this kind of backsliding happens very quickly and drastically. When finally new technologies are required for your projects, it could be embarrassing that your development skill is much like how it were at the early stage of your profession.
Having your personal projects not only mean that you build something, but also that you make own decisions. Because you are in charge, you can test your implementation skill with new technologies without waiting for anything. Whether your attempt to use new ideas is successful or not, you will be prepared to deal with issues later when it’s much needed for real jobs assigned by your employer.
Along with technical decisions, you can also make scheduling one on your own, without any constraints. Of course, once it’s public and used by many others, you may have to listen to their requests and satisfy their demands as soon as possible. However, it’s done proactively, being able to lengthen or even shorten the development time as you like. This sense of controlling time is probably only possible if you have complete ownership of your project, which is by having personal project outside of your job.
Also, no matter how much dedication you’ve put into your works, technically these jobs assigned by your employer are actually not yours. These useful evidences for proving your skills may not be permitted to be used when you are applying for a new job from another company. But with you own personal projects, you can just submit them to be fully analyzed without any guilt but with great confidence. It’s always better to silently show what you are so good at, than just loudly speak about it.