This is the big reason. I keep the boxes to make future moves easy, learned the lesson the hard way.

Other good reasons are the box keeps all the accessories in one place while saying what they are for (psu boxes, looking at you) or the box is great at storing that type of item, like the box my NAS drives came in, perfect foam insert for other drives

A lot of people start on used small form factor desktops like the ASUS PS52. Other common ones are the Lenovo M series tiny desktops, Dell Optiplex micro desktops or Intel NUCs. These can sometimes be found used for sub $100 a piece from businesses updating their fleets. They can struggle a bit doing stuff that needs some CPU grunt, like live plex transcoding, but are decent otherwise, especially considering the low power draw. You might want to consider spreading that software demand over two or three of them.