Setting CPU affinity in SQL Server is not a task you do every day. Rarely are there use cases when you need to do that. I had a recent requirement to do it. We plan to replace a physical server with half of its current CPU. Primarily due to faster CPU and workload moved off of SQL Server to other cloud services. To test, we needed to set the CPU affinity mask in one of our non-production servers. In the research, I learned about the side effect of setting CPU affinity mask, which is nicely explained in this ( by Klaus Aschenbrenner) and this (by Adam Denby) blog post.
Click through to learn more about the process.