When a company goes bust, its servers are switched off, and as far as the internet is concerned, your domain name no longer exists. If the website cannot be reached the emails do not work any more either. For an internet business this could be catastrophic. And, even for owners of only one domain, months of time, energy and money spent creating the site have been wasted.
This is why it is important to make sure your name is on the whois at your ISP email address. It is tempting to use a cheap email address to thwart spam (I do, and I lose alot of sleep over it). Atleast if the host goes doa, you can still get to your domain whois and update the tech info to switch your ip. You can't do that if you are dependant on another email address that just died with the host.