I have an Ubuntu Server set up that shares files online, has a web server, streams to PS3, iPad, etc.
I also recently acquired a Raspberry Pi with Raspbian installed.
What exactly is the difference? What I mean is:
The Raspberry Pi was designed as a low power computer.
Low power both in power usage, and cpu workload. (I'm not counting GPU power here)
It'll be able to do a lot of what your Ubuntu server can do, but at a lower power usage, and most likely a bit slower.
The main things that the Pi has over a computer, is the easily accessible GPIO ports, and it's physical dimensions.
You can inteface with motors, buttons, etc far easier on a Pi then you could on a normal computer.
You can also cram the Pi into much smaller spaces than you could with a more powerful computer.
1.
If you want to use the Raspbery Pi instead of a real server then No, you wont have any use for the Pi because a x86 based computer is faster and more robust than the Pi
2.
It is not essential for anything. It is mostly for experimentation.
You yourself might have brought it out of curiosity so go and try to build a remote garage door opener using your mobile phone over bluetooth.
3.
Simple answer is No.
Server is best run on a full processor computer with loads of RAM and proper interfaces like PCI-Express, real LAN port*s*. The Pi can run it but it will only serve you well and hardly cope with several users.
For the most part, the Pi is just an ultra cheap, ultra small, ultra low-powered system. It runs a specialized version of Debian which is the Linux distro Ubuntu is based off of so there's a lot of similarities. In general, unless you're wanting to run ARM assembly code, your full-sized PC will be better. The Pi was designed with the intent of providing a cheap computer for teens to learn to code and use Linux on. There's not much "special" about it other than the price.
Apart from the GPIO ports and the nifty ability to use an old TV as a display, the big thing the Pi has going for it is that it is totally quiet and consumes very little energy. A full-sized PC will normally have a noisy fan, put off a noticeable amount of heat and consumes (when sleep is disabled for server use) a noticeable amount of power. If you have a small home/office that needs a print server and/or a low-traffic file server, a full fledged PC is kinda overkill. If this sever won't ever have much traffic, a Pi can be run 24x777x365 with a negligible impact on your electric bill.
