Giving users what they are used to: More than half the world prefers Google's search results and are comfortable interacting with the search box as well as the search results. Google Custom search engine provides the exact same search experience as Google.com but on YOUR site.

Building your own search engine is difficult: Have you ever tried designing your own site search functionality, it's definitely not simple. It's difficult to apply the same type of algorithms Google does so that you can sort your results smarter than just alphabetically. With the custom search engine the algorithms are applied for you and you gain all the benefits of the awesome performance.

Access to deeper content: Many sites on the Internet have some of their best content buried deep within their folder structure, the custom search engine allows your users to find that deep content within 1 click without getting frustrated and abandoning your site.

Access to new content, not a huge issue anymore: One previous draw back of Google Custom Search vs. a proprietary search was that Google took a little while longer to index newer content which made a given site's internal search results not as fresh. However, with the recent changes, your new content can be made available quicker by leveraging Google's XML sitemaps.

Customization: Google Custom Search Engine allows you to customize your configuration in many different ways including: look and feel of results, style of search box, placement of ads, sites to search within, ability to introduce AdSense, method of displaying results and refinement labels for popular website topics.