You hire the service of the web host to get your website hosted on his server. So, one leases the services of the web hosting service provider. If you are content that you have the online presence and that you are making your web site accessible to the traffic.
But the task is not completed here; the web owner has to be there, to check whether his web site is running properly. It is his responsibility to ensure that his web site is in the accessible condition for the visitors. Sometimes the problems arise in the website, the server may be down or the transfer may take more time etc. such technical problems are sure to occur. And web owner may not be in the position is look after all these and call up the service provider.
One need not get worried about this, as the option of managed hosting service is available. The managed web hosting offers this convenience to the web owner. He will not be required to spend time in monitoring the web site, as this service is also performed by the host himself. Thus the managed web hosting is widely advised.