A complete empty article which could be summed up by "make an API because it will be useful for your users, p.s. if your service doesn't need an API then don't make one".
The API is the new embodiment of the "If you build it, they will come" myth, and this post buys into it big time.
And I once again fell for the myth that titles starting with 'why' and not asking questions are interesting...
Except in all the cases where an API doesn't make sense for your startup or isn't important at the beginning.
And then when you've finally built an API you can distribute it on http://mashape.com to get some traction and five auto-generated client libraries for free