Approved scanning vendors are the companies which are regulated by the PCI Security Standards Council who are the governing body for online payments. The Approved scanning vendors supply scanning services to merchants to ensure their severs are secure and hacker safe.
It is very important if you are accepting credit cards online to ensure your servers are PCI complianant else you could very well be risking your customers card details and as seriously find yourself hacked with your clients data lost causing your company great harm.
There are many breaches of security and depending on your business model such events can ruin your companies online reputation.
There are always new security threats which are constantly causing problems, therefore the ASVs have to keep up-to-date with these problems and ensure the scanning services capture all the known vulnerabilities with the ever increasing release of new software.