Shutting down your online business for any amount of time is every owners nightmare - every minute spent offline could potentially be a client lost to your competitors. Small security bugs could take your site out for a few hours, but a larger hack could force your website offline for days while diagnosing and cleaning up the problems.
The more worrying thing is that it's not only hackers that are after your websites data, but recently spammers have been using more advanced tactics recently to find ways to exploit code in order to insert their links within your site.
Web coding is a fast moving industry, and as more overall improvements to code become more widely available, there can be small bugs in the code which hackers can exploit once they find vulnerabilities. Staying ahead of the hackers by keeping up to date with patches for security flaws is crucial, as with the tools hackers use, they operate on a mass scale and specifically target older versions of site frameworks with known weaknesses.
Constant security updates aren't the only thing to consider though, here's a quick list of other things to consider when improving your overall site security, and things you can implement right from the start.