The key advantage of shared hosting is cost. Given that a number of websites share a server, the cost of maintenance is shared between a number of accounts. This brings end user costs down considerably. A good web hosting account offering 1GB of disk space and similar amounts of data transfer can be available for 12EUR a month.
Another advantage is that as the web host is responsible for managing a server, support is available if issues occur with the server. This means that people with relatively few IT skills can still set up a hosting account and a web site.