This is an important new feature that makes Google Custom Search more useful. You can use Google CSE to provide your users with a way to search only a short list of sites.
On-Demand indexing allows anyone with a Google Custom Search Engine or Google Site Search to identify and tell us about new pages or recent site changes by submitting a Sitemap to our Webmaster Tools.
Google Custom Search: On-Demand Indexing for a fast-moving web
This is only really useful for large sites with a lot of content. I would argue that for small sites and blogs, the normal global Google search is the only way people are going to find you any way, and secondary searches on your site are pretty small secondary source of traffic. Lijit, which is a service organizing Google CSE for bloggers is pretty useless, IMHO. When developers have full API access to Google CSE, it becomes a lot more interesting. This would surely be something developers would pay for, as the only alternative currently is Yahoo BOSS.
