When you submit a sitemap to Google Webmaster Tool, then web crawlers finds your website, crawls and reads everything of your website and then save it in its database. It is important in SEO to let know web crawlers about your site.
The way in which spiders can find your website: As i know as analysis-
Spiders can find your website through search engine submission and sitemap submission in webmaster tool.
They can also find it through the tracking code of search engines that are inserted in your website pages. etc.