Just that you don't need a beast of a machine (with it's higher cost and power consumption) to just serve files at reasonable performance. If you want to stream video, you'll need greater performance.

For example, my NAS is ten years old, runs on ARM, with maybe 2gigs of ram. It supposedly can host services and stream video. It can't. But it's power draw is about 4 watts at idle.

My newer (5 year old) small form factor desktop has a multi-core Intel cpu, true gigabit network card, a decent video card, with an idle draw of under 12 watts, and peaks at 200w when I'm converting video. It can easily stream videos.

My gaming desktop draws 200w at idle.

My SFF and gaming rig are both overkill for simple file sharing, and both cost 2x to 4x more than the NAS (bought the NAS and SFF second hand). But the NAS can't really stream video.

Power draw is a massive factor these days, as these devices run 24/7.

RPi is great for it's incredibly low power draw. The negative of RPi is you still need enclosure, and you'll have drives that draw power attached to it. In my experience once I've built a NAS, RPi doesn't draw significantly less than my SFF with the same drives installed, as it seems the drives are the greatest consumer. As I mentioned, my SFF with 1TB of storage draws 12 watts, and RPi will draw upwards of 8 watts on its own (my Pi Zero draws 2, but I'd never use it for a NAS). It's all so close that for me the downside of RPi isn't worth the difference in power.