There are lots of reasons that sites get hacked.  Of course there are the obvious reasons such as that the hacker wants to steal from the monetary aspect of your site but most hacking goes far beyond this.

One example that any Australian will know about it the Census 2016 debacle. Hackers place malware on sites, usually done through software vulnerabilities and then at a later time that they choose will activate the malware to take control of all the sites. They then use all the sites they have hacked to attack the site they are wanting to take down. This is usually a very large site like a Government site or a huge international corporation such as Facebook.  

They just use your site as collateral for the bigger attack.  In doing this your site will become flagged with Google as hacked and will be removed from Google ranking until it is clean.  

You may also have your site destroyed beyond repair and without quality backup and restore options in place you will need to have your site rebuilt from scratch.  Not only is that a very costly experience but you will be without your site for 4 – 8 weeks.  Can you afford that type of downtime?