Arduino is a platform (the boards, the IDE, the language, debugging tools, etc) that allows any sort of user new to programming and/or hardware to get started very quickly. It is a great gateway into raw microcontrollers, as it is based on the Atmel/Microchip ATMega line of chips, which are also fairly easy to program. Most people can easily transition into that and build "production-ready" things.

Also, many 3D printers, laser cutting tools, and CNC routing tools use Arduino as their microcontroller of choice. It represents a huge community and is one of the most basic of tools in any new or seasoned maker's toolbelt.