Once you've uploaded code, you can inspect and change variables (including functions!) while your program is running. There's loads of documentation, tutorials and support for a huge range of different hardware too. See how to get started here, or if you have any questions ask away on the Espruino forums!
The Espruino's fast response time has a lot of advantages. It allows for quick and easy debugging and is a great way to test your project before your big reveal. In addition, you can control the Espruino from almost anything - Windows, Mac OS, Linux, RasPi, Android, anything that can talk to a USB Serial port.
The Espruino also interacts well with our NeoPixels. For more info, check out Espruino's page on the WS2811 and WS2812.
While the main advantage of the Espruino is its instant execution, it can also be used as a traditional board through a Web-based IDE hosted on your computer. The microcontroller also uses less power than Linux Boards (although its of course a lot less powerful as well) so will run longer on battery power, it has loads of IO pins, and it can be used as an IO board for PCs, Macs, or Rasp Pis without having to program it first. Simply take the Espruino out of its packaging and get started!
There's also more info on the Espruino page including tutorials and code examples. There aren't currently many tutorials using this board, however it can be used just like the Espruino Pico, which has a lot of tutorials available.