Airsoft gives people the ability to own a shooting version of their dream gun. Due to the law and the cost, people cannot always own their dream gun, and Airsoft provides you with a realistic replica, and on top of that allows you to be able to shoot that replica (albeit: non-lethal).
Often, people would watch movies and TV and say, "Wow, that gun is awesome, I'd like to get me one of those". Actually, what you may be seeing in those movies and TV shows, ARE, in fact, Airsoft guns. Due to their relatively cheap price tags and precise realism, movie studios often employ Airsoft guns for it's production work. They simply digitally alter the images to incorporate the muzzle flash, sound and effects to mimic the real firearms.
Generally speaking, most of the more popular real-steel firearms has an Airsoft gun counterpart. The MP5 series, to include MP5A4, MP5 SD5, MP5 PDW. The venerable G3 Battle Rifle series. Colt M16s. Steyr AUGs. SIG 550s. FAMAS. AK-47s. Machine Guns like the M60. Sub-machine guns such as the IMI Uzis and Ingrams. Pistols such as the famed Berettas, Colts, SIGs, and Glocks. Shotguns such as the Benelli M2 and Remington 870. Sniping rifles such as the PSG-1, M40A1, Blaser 93R. Even the "Hollywood" guns like the RoboCop auto-pistol and the M134 Vulcan (6-barrel gatling gun) made famous by the Predator and Terminator movies. The list of Airsoft guns is quite sizeable, and growing with each passing month.
The ability to own and play with these "dream guns" are in and of itself appealing enough to enthusiasts. The added advantage of being able to take these guns out to a local playing field and "shooting" your best friends with it in friendly competition is a hands down selling point.
Another appeal with Airsoft is the ability to "role-play" with them. There are countless clubs, teams and organizations devoted to the sport of "waging war" with airsoft guns. Known throughout the world as Skirmishing. In essence, role-playing allows people to fulfill their fantasies, which (due to physical disabilities or circumstances) they never got the chance to. These people may be bankers, insurance brokers, secretaries, students, computer programmers by profession, but for 1 day a week they can be Green Berets, Assassins, SWAT members, Navy SEALs, whatever. Their mind is the only limit. This role-playing ability is enhanced by the fact that Airsoft guns are extremely realistic. After all, it's rather difficult to play the part of a Navy SEAL when you're holding a neon, orange-yellow-colored, Super-Soaker squirt gun.