Signing documents is good idea but why I need GenuineDoc?

Ease of use. It is true that digital signature verification can be performed without a third party such as GenuineDoc. But with GenuineDoc, your customers don't need to download any software to verify HTML pages, they just click a link and GenuineDoc verifies the signature, all is done within their browser.
Flexibility. You are not forced to sign or fingerprint the whole document. You can sign just the content that matters, leaving all design elements outside the signature. See our Terms page as an example, note that we signed only the terms themselves leaving out all menus, logo, etc. This way, if you later want to change the look and feel of your site, you can do it without breaking your old signatures and having to re-sign and re-timestamp everithing. Your old documents will never look old-fashioned, they will fit into any design. The need for flexibility is immediate when you publish e.g. a press release which is to be copied to many news sites with their different designs.