If you link to your sitemap from your website they will find it yes... Although many don't like to do that (me for one) for several reasons (such as):
You might not want competitors (or surfers) to find your sitemap and discover your full website architecture (you may not want them to know every url on your website).
If you want to make sure that SE's will follow the sitemap link, you need to place the sitemap link on a prominent page... Which means "wasting" a link from an important page (even if you nofollow the link it will still "cost" your other outbound links some equity).
If your sitemap links to your pages (as opposed to just listing them) you are "flattening" the architecture of your site (especially with a link from a prominent page, or - worse still {and quite common} a link from every page)!
When you supply the sitemap url in your robots.txt you don't "waste" any "link juice" (page rank) on it (or equalise the PR flow around your website).