First of all, the sheer volume of members at these websites drastically widens the pool of possible candidates.While quantity of members is not nearly as important as the quality, the best dating sites provide filters to make sure you meet quality people.
However, the world is a very large place and the chance of encountering your soul mate in your day to day life is small, especially if you are busy or overly shy.
Dating websites are especially helpful for people who are shy or nervous; there is much less pressure involved in sending a message than approaching someone in person.
Online dating offers the benefit of a relaxed atmosphere, where you can take time to think about what you want to say without nervously blurting out the first thing that pops into your head.
The second is something you may not have thought of.
People may hold back feelings or thoughts when meeting in person because they are too embarrassed to talk about them, but when people write online they tend to disclose more about themselves, which lets you get to know them better.