1) Because you have access to millions of people; you know they are looking for someone (and exactly what they are looking for); you can usually see photos of them; you can find out their likes and dislikes, their age, and their physical characteristics; you can communicate with them; AND you can do all this anonymously. You can search for matches by whatever criteria YOU decide. That's a heck of a deal for a usually quite nominal fee.
2) Because it works. Thousands of people have found mates online in the last few years alone. That number is increasing in frequency as online dating gains more acceptance. Once regarded as a venue only for losers and the truly desperate, online dating has gained respectability through all walks of society.
3) All those shy individuals (especially women) who would never feel comfortable approaching you in a public place will feel brave enough to contact you in this environment.
4) Couples who develop a relationship online tend to reveal themselves to one another over time without the overemphasis of physical appearance and complications of other distractions; i.e., we were friends first..... They have developed the art of communicating with each other. Also, a person will almost always reveal more in an email than they would face to face.
5) Because these relationships are founded in compatibility, they tend to have more longevity once established.
All in all, while you still may have to kiss a lot of frogs to find your prince, online dating is a definite must-try for anyone serious about looking for love.