Why we use 1U servers, and the two sides of them
Every so often I talk about '1U servers' and sort of assume that people know both what '1U' means here and what sort of server I mean by this. The latter is somewhat of a leap, since there are two sorts of server that 1U servers can be, and the former requires some hardware knowledge that may be getting less and less common in this age of the cloud.
In this context, the 'U' in 1U (or 2U, 3U, 4U, 5U, and so on) stands for a rack unit, a measure of server height in a standard server rack. Because racks have a standard width and a standard maximum depth, height is the only important variation in size for in rack mounted servers. A 1U server is thus the smallest practical standalone server that you can get.
(Some 1U servers are shorter than others, and sometimes these short servers cause problems with physical access. They don't really save you any space because you generally can't put things behind them.)
In practice, there are two sorts of 1U servers, each with a separate audience. The first sort of 1U server is for people who have a limited amount of rack space and so want to pack as much computing into it as they can; these are high powered servers, densely packed with CPUs, memory, and storage, and are correspondingly expensive. The second sort of 1U server is for people who have a limited amount of money and want to get as many physical servers for it as possible; these servers have relatively sparse features and are generally not powerful, but they are the most inexpensive decently made rack mount servers you can buy.
(I believe that the cheapest servers are 1U because that minimizes the amount of sheet metal and so on involved. The motherboard, RAM, and a few 3.5" HDs can easily fit in the 1U height, and apparently it's not a problem for the power supply either. CPUs tend to be cooled using heatsinks with forced fan airflow over them, and often not very power hungry to start with. You generally get space for one or two PCIe cards mounted sideways on special risers, which is important if you want to add, say, 10G-T networking to your inexpensive 1U servers.)
We aren't rack space constrained, so our 1U servers are the inexpensive sort. We've had various generations of these servers, mostly from Dell; our 'current' generation are Dell R230s. That we buy 1U servers on price, to be inexpensive, is part of why our servers aren't as remote operation resilient as I'd now like.
(We have a few 1U servers that are more the 'dense and powerful' style than the 'inexpensive' style; they were generally bought for special purposes. I believe that some of them are from Supermicro.)