The aim is to influence customers to use your product on their own, again and again without relying on overt calls to action such as ads or promotions.

User habits are a competitive advantage. Products that change customer routines are less susceptible to attacks from other companies.

Users who continually find value in a product are more likely to tell their friends about it.

Why not live now instead of someday?

When designers intentionally trick users into inviting friends or blasting a message to their social networks, they may see some initial growth, but it comes at the expense of users' goodwill and trust. When people discover they've been duped, they vent their frustration and stop using the product.

Reducing the thinking required to take the next action increases the likelihood of the desired behavior occurring unconsciously.

To initiate (user) action, doing must be easier than thinking.