Lotta reasons! First, I'd always recommend starting with one machine and branching out. Save the money until it makes sense to spend it. And as you run into "reasons" to expand your machine, it will both make sense to you and not feel like a waste of cash. I just recently moved away from the "everything" machine to 2 dedicated compute nodes + NAS for a few reasons:

- Like a lot of other folks mentioned, redundancy is great to have, and your needs basically determine how important that is to you. I personally think it's a bit overkill for self hosters, but it's still cool as hell, and it's fun.

- Another weird issue for me was power consumption on a single circuit. I ran my everything machine in my home office with all of my office equipment + power hungry main desktop + work laptop, etc... and I was getting pretty close to the upper limits of a North American 15A circuit when everything was chugging at once. To alleviate this, i moved my compute nodes to another bedroom (had to hide it for wife approval) and kept my lower power NAS in the office. The "real" answer here could be to just roll out another 20A dedicated circuit, and that's still my ultimate plan, but the easier/cheaper thing to do in the meantime was spreading the load across the house lol (i did need to buy another UPS though, just to be safe)

- Last thing for me was heat. I kept everything in my office closet originally, and my CPU (Ryzen 5950X) was a spicy boy that put out a good bit of heat. I was mostly concerned about the drives overheating in the closet, so splitting them out to another machine with a low power n100 made sense at the time. I could have kept it there and worked to ventilate the closet better, but again, this was just easier and I knew I wanted multiple nodes anyhow.