ESTA and API are different things.
Of course you're right, but I think we know what the OP meant. They are both collecting the information (I think, from what I've read, almost exactly the same information) but for delivery and processing at different times.
The advantages of the 72 hour advance information - although everyone seems to be complaining about it - are:
- It should, theoretically, speed up immigration at the airports by some margin, since the Immigration officials have part of their burden relieved - if you've been cleared by the computers already, they're less responsible for vetting. At the moment there is no guarantee that the API has been processed by the time a flight arrives - the 72 hours (increased from the initially-proposed 48 hours) gives an idea of how long it can take.
- The ESTA lasts for 2 years, I think (or was it 3? whatever...) so less work than filling in API and green forms for each trip.
- From the airlines' point of view, there have been occasions where they've sent the info off to the US as the flight finishes checking in and then have been told to turn back or stop en route by US ...the plane already on its way by the time the processing was done. A
famous example was when the singer Cat Stevens was refused entry and his plane had to divert, I think. This saves the risk of that enormous expense.