1. You probably would want people visiting your website to be able to click through to your social media channels, whether thats your Facebook page, G+, Pinterest, Twitter or all of them. Or any of the many other social media channels. You may update your Facebook Page or other social media and not your website, so this maybe important to you.
2. Do you want your website to have Facebook 'like' or 'share' buttons? Or other media sharing?
This can help with 'engagement' and help make your site interactive but it is not a given.
3. You may want your Facebook Page and updates to appear on the website itself thus integrating totally. This can mean less updating on your website.
There is no hard and fast rules with social media integration, generally most websites do have at least social media links. it really depends on the nature of what your business is and how quickly you update things and where you update.
At Litebulb we generally include the setting up of a Facebook Page as well as a Google Maps Business Listing and G+ if required.