Paid inclusion options tend to offer other perks as well, such as guaranteed revisits to update your listings (e.g., every 24 hours), guaranteed inclusion on any partners' Web sites, reporting to track your performance, and in some cases a review of your Web site to ensure its relevance. With paid inclusion you are guaranteed to be indexed by the search engine, up to the number of pages you have paid for, within a short, defined period.
Certainly, you might try to "ask" Google to crawl your website, but that will put you in a looooong list of websites hoping to get added. The hands-down most effective way to get added quickly is to have a link to your website - somewhere. Now that someone has linked to your website, and Google stumbles that link, your website will be queued for fast-indexing.