Another reason (besides the ones already mentioned in the other comments) is that some imports might only be available on certain operating systems or architect

Another reason (besides the ones already mentioned in the other comments) is that some imports might only be available on certain operating systems or architectures. I once wrote heavily optimized ML code for Nvidia Jetson Nano devices but I still wanted to be able to test the overall application (the non-Nvidia-specific code) on my laptop or in pipelines.