Google is the best source for traffic to your website and if your site is banned by Google, You will lose that source. Sites which are banned by Google may not appear in search result pages. There are many reasons for which Google ban websites. The sites which caught violating any of the rules of Google, get banned by Google.
Rani Dababneh has explained it well in his post – Penalized by Google is worst than being Dumped by your Girlfriend!. Read his post and you will also realise that Google and Girlfriends are almost common. He has also explained some points that you must follow for not being banned by Google.
Link : Penalized by Google is worst than being Dumped by your Girlfriend!