When out server shopping, you may come across many tower servers or 1u servers. Both of these have their own specific purpose but are generally used as data handling machines.
1u servers are extremely micro, slim and prebuilt server systems which are built to be fit into a rack mount. The advantage of these is that you can fit many of these into a smaller space as compared to typical rank mount servers.
They are also used commonly for mass storage solutions or fast storage solutions.
You can also order these kinds of server by specially ordering them from manufacturers like Dell. Although they are less powerful as compared to their full size counterparts, they have the advantage of size.
Tower servers are also very common, they are almost like typical consumer grade PCs which you may have in your home. But, they are designed to perform different and specific tasks.
Data handling, server hosting, data base management and so on are the typical things which a tower server can be used for. These servers are also quite popular because they can essentially be used like a normal computer, easy to maintain and see if there are any problems.