There are few developers who give themselves the luxury of never programming outside of work time. Indeed most developers, even if trained in some part academically, will have learned most of what they know working in their spare time, whether early on as a student or whilst working in another profession. The ability to code, and to produce good products, rarely comes without significant amounts of our own time invested. Neither is this time usually given grudgingly - as we develop our skills we increase our power to create, to use our imaginations to invent “products out of sheer thought stuff” as one famous software engineer put it. For those looking to hire employees into any software development team, whether for a large or small company, people who spend at least some time working on code which they are not obligated to work on are often preferred - those people may be ahead of the curve on new technologies, have better subconscious knowledge of their favourite languages and more easily recognise good from bad practice. As developers, working on our own projects is unquestionably a good thing.
Another way that developers can end up working alone is in the bold new world of freelance - coders without an office, a boss, a pension or even, sometimes, a pair of shoes. Whilst many freelancers end up being brought in to support an existing team, or even as pseudo-consultants, tasked with more responsibility than just raw programming, others will find themselves working on totally new projects. These may be existing companies trying out a new idea, or we may encounter founders of a startup business who are unlucky enough to have a great idea and no way to build it themselves. These projects straddle the line with much of the creative freedom of our own projects but also additional financial reward, and possibly more in the way of penalties for doing “cowboy” work, writing bad code that someone else will have to pick up in future.
For some of us our personal projects may become more than just a way to hone our skills, try something new or produce a product for personal consumption - they may become businesses in their own right. For the freelancer striking out with new ideas for an existing company or helping develop a Minimum Viable Product (MVP) for a startup, the business case might actually work and your experiment is used by internal staff or external customers of the business. At these points you might be set for an interesting trajectory, as requirements and pressures necessitate more hands on deck to keep building this product - the challenge of going from one to many.