The SOAP search API allowed you to obtain search results and integrate them into your applications. You could develop applications like Google Share that measures the popularity of an item within a domain (for example: Bono's Google Share for U2 is 54.6%), create a meta-search engine by mixing different APIs and more.
Now Google suggests using Ajax Search API, but this is very limited, it's suitable only for web applications, you can't reorder the search results or add other search results. Google also says: "AJAX Search API is the only permissible way to publish Google AJAX Search API results on your site. We'll block your application if it accesses search results outside of the API."
Applications that already use the SOAP API can continue do that, but the service could become unreliable.
O'Reilly Radar says: "The AJAX Search API is great for web applications and users that want to bling their blog, but does not provide the flexibility of the SOAP API. I am surprised that it has not been replaced with a GData API instead."
Nelson Minar, who authored the API, has an explanation: "It seems like good discipline to me; when your corporate culture has a "go fast, do a lot of things, fail often" approach to product development, you have to do something with the things that succeeded in launching but then failed to make a big impact on the business."
Google Search API has been launched in April 2002, one week before Google Answers, that was also discontinued.