Why use a Google Sitemap?

I've played around with Google Sitemaps on a couple sites. The lastmod, changefreq, and priority parameters are pretty cool in theory. But in practice I haven't seen these parameters affect much.

And most of my sites don't have a Google Sitemap and that has worked out fine. Google still crawls the site and finds all of my pages. The old meta robot and robots.txt mechanisms still work when you don't want a page (or directory) to be indexed. And I just leave every other page alone and as long as there's a link to it Google will fi开发者_JAVA百科nd it.

So what reasons have you found to write a Google Sitemap? Is it worth it?

From the FAQ:

Sitemaps are particularly helpful if:

• Your site has dynamic content.
• Your site has pages that aren't easily discovered by Googlebot during the crawl process—for example, pages featuring rich AJAX or images.
• Your site is new and has few links to it. (Googlebot crawls the web by following links from one page to another, so if your site isn't well linked, it may be hard for us to discover it.)
• Your site has a large archive of content pages that are not well linked to each other, or are not linked at all.

It also allows you to provide more granular information to Google about the relative importance of pages in your site and how often the spider should come back. And, as mentioned above, if Google deems your site important enough to show sublinks under in the search results, you can control what appears via sitemap.

I believe the "special links" in search results are generated from the google sitemap. What do I mean by "special link"? Search for "apache", below the first result (Apache software foundation) there are two columns of links ("Apache Server", "Tomcat", "FAQ").

I guess it helps Google to prioritize their crawl? But in practice I was involved in a project where we used the gzip-ed large version of it where it helped massively. And AFAIK there is a nice integration with webmaster tools as well.

I am also curious about the topic, but does it cost anything to generate a sitemap?

In theory, anything that costs nothing and may have a potential gain, even if very small or very remote, can be defined as "worth it".

In addition, Google says: "Tell us about your pages with Sitemaps: which ones are the most important to you and how often they change. You can also let us know how you would like the URLs we index to appear." (Webmaster Tools)

I don't think that the bold statement above is possible with the traditional mechanisms that search engines use to discover URLs.