It is likely in relation to running macOS on non apple hardware, as the Darwin kernel is often distributed with hackintosh tools. This would give them a reason to send a cease and desist otherwise legal software.

edit: wording

Yes, in many cases. Building is very CPU and memory intense. Many times you have to run local severs to debug on. I often find myself running local servers, building my apps, running my IDE, creating assets in Photoshop, and more all at once.