“Turning off” macintosh.js from its file directory saves your virtual desktop, so you won’t lose your fancy drawings or word documents. You can even transfer files between your computer and the macintosh.js emulator, or mount a disk to the emulator for a hardcore file-transferring experience.
According to Wikipedia, the Quadra 900 launched with an introductory price of $7,200 in 1991. That’s around $13,600 if you adjust for inflation. Considering that the free macintosh.js comes with a copy of Photoshop 3.0.5, it’s quite a good bargain.