Have to agree with Camerongray. It will always depend on what you intend to do with it- there isn't really a Single-board computer for people who simply *want* a single-board computer- If nothing else you would obviously want to do something with it.
As an example a Raspberry Pi could conceivably be used to provide functionality to a TV in the sense that you could mount it "on" the TV and plug it into the HDMI port and supply power from a USB Port; you turn on the TV, you turn on the Pi, and can simply switch to that HDMI port. With the typical "NOOBS" Linux distribution you could have things "always available" on the TV by switching to that HDMI input; viewing youtube videos, watching movies/TV Shows, or even playing games. Paired up with wireless peripherals there wouldn't even be wires coming from the TV. Some of that can be done with so-called "Smart" TVs however Smart TVs also have many privacy implications- many manufacturers have been found collecting usage information as well as personal information and sending it to their servers, for example, without any explicit consent.
Another application would be to set up a "Pi Hole"- I have one set up for this right now. It effectively blocks ads and certain domains/info by acting as an "in-between" DNS Server. I have my router configured to use the Pi as a DNS server, which in turn will filter out certain requests for known ad domains altogether and otherwise forward the request to a "normal" DNS Server, meaning that I get some set of ad blocking features across my entire network on every device.
And as Cameron notes they aren't the same as microcontrollers like the arduino, which are more like components that would be part of a larger project than something that can be used more or less independently.