Once you've uploaded code, you can inspect and change variables (including functions!) while your program is running.
The Espruino Pico is designed to be easy to include in your designs. The 0.01" pins are easy to fit in to sockets, and castellated edges mean that unpinned Picos can easily be surface-mounted directly to a PCB.
This is the pinned version of the board, which fits perfectly into breadboard. Want the unpinned version.
I was amazed at how difficult it was to get started with (and then to debug) most Microcontrollers (especially ARMs), and the lack of support for different platforms. I thought that by putting the development tools on the device itself I could come up with a microcontroller board that could be programmed easily from almost any kind of computer.
Not only that, but your source code is on the board itself. If you make something with an Espruino board and need to change it a year later, your original code is still there waiting for you!