Workstation (to me at least) usually means a high-end PC geared specifically for a few work-focused tasks like CAD or Excel. Often they use high-end and multi-socket server class processors, large amounts of error correcting RAM, "workstation" class graphics cards from the Nvidia Quadro and AMD Firepro series, high-end SSDs and fast connections to backup servers.
As to why you would buy one, this kind of kit can let you do your work faster/better and time is money. If you don't plan to do work on them then steer clear of them - they are not suited for home/gaming systems and generally very expensive.
