If you don’t reduce it, you’ll do your work on very very big pic, it means, big image size and, of course, very big file size.
If you have very cutting-edge graphic environment and huge memory, you don’t need to care, may be. But, this software actually requires quite strong GPU environment, as you know, maybe... If you don’t have good GPUs, this software works very very heavily, and sometimes(or often) freezes.
(Generally, GPU-based softwares are less stable than CPU-based. And, computer hardwares are basically optimezed for CPU.)
It’s my very personal opinion. Try it by yourself !
(Imagine... when you use Photoshop, for ex., and duplicate based layer again and again, its file size becomes bigger. 300x300 pxls pic is smaller than 1000x1000 pxls pic..., etc...)