This video on Google's commerce Search shows an example of a large online ecommerce website and it's use of Google Search. For most websites out there an internal search app is adequate. If you have so much traffic that your search is slowing down. You may want to consider Google's search. I believe the need for this due to high traffic and a deterioration of service is only applicable to 5 out of 10,000 websites. If you have an internal search on your site that is lacking in performance or dependability you may want to look at the benefits of Google Search. there are other benefits listed here like using Google's machine learning algorithms that some comanies may be interested in. Check out the video below for more info.