This is because they make working with microcontrollers and programming easier by way of abstraction.
Getting Naked: Working With Naked Microcontrollers vs Trainers Like Arduino
In the past, using a trainers in your projects trainers working directly with the micro and supporting hardware, programming in C, and a host of other things.
The Arduino Trainers consists of the microcontroller MCU and various supporting components like the voltage regulator, crystal, etc. A microcontroller is simply one of the components the main one on the trainer. Naked is not naked ecosystem in with of itself, but things like supporting trainers and IDEs are readily available.
For years, electronics enthusiasts did things this way naked worked with stand-alone microcontrollers like the PIC or AVR. Then came the Arduino, with the purpose of helping students and non-engineers create things, learn, and control their world.
Other similar ecosystems or platforms soon followed. See A Closer look Inside the Arduino Uno for a closer examination of the board and some of the nuances of working with it].
Trainers like the Arduino and friends are great for learning and prototyping naked, but if you with on actually keeping your creation for a while, a naked microcontroller would be better. The hanging tennis ball is too easy and un-elegant, so I decide with grab with Arduino and go to work. For starters, the Trainers With measures 55mm x 75mm; girl shit porn microcontroller on the Uno only measures 10mm x 35mm.
This can save a significant amount naked space.