When you run the custom tool of a T4 Template, it generates code in a child node of that template. On StackOverflow I have seen a a lot of devs asking how to write to separate files, or even to a different folder. This is actually easy to achieve, and there are multiple possible solutions.
This is not so hard to do actually. It ensures your transformations are executed when you build your solution, when in some cases you did not click ‘Build > Transform all T4 Templates’ when changes are made to your .tt files. If one of your projects uses T4 to generate code, and you want it