This is a key to emulation history. A lot of arcade machines no longer work and with the rise of the video game consoles with Internet access nobody plays arcade games anymore. With MESS a lot of the old video game consoles and home computers are emulated even if they are no longer supported or made anymore.
With the JavaScript versions of MESS and MAME once they finish them you can play all of those games in the web browser.
ROMs and Disk Images can be found on web sites and BitTorrent sites, but be aware some of the DOS floppy disks and EXE files might be virus infected, hacked, or modified in some way to make a crack work. I say this as a warning because when people archive these DOS games they don't check for viruses and other stuff. In QEMU there is a Venom vulnerability for floppy drive emulation: http://www.phoronix.com/scan.php?page=news_item&px=QEMU-VENO... there might be a similar one in MESS.