The original consoles/computers are fantastic for the nostalgic purpose...but unless you are a hardcore collector quite impractical to have a room full of old, often unreliable gear to play some classics. Imagine going older than MD cartridges...how much fun is it to still load ZX spectrum games from tape (if the magnetic software still works).
This is where emulators are great. Imagine playing older arcade classics without MAME, needing a warehouse of old unreilable machines when MAME gives you the biggest 80s arcade you saw at your fingertips! ;-)
As I mentioned, I have one of those cheap Blaze Handheld Megadrive units, with an SD card covering the MD library. It's not quite as good, but does the job without space or reliability issues! Streets of Rage 1 or2... The Road Rash series, Ghosts n Goblins, Sonics...bam! In your pocket, ready to go without digging out consoles or carts and getting them to go!