There are quite a few advantages to be realized with shared hosting. This is almost always the cheapest type of hosting that is commercially available as the cost of the server space is usually made up for by sheer number of customers. These customers will not have to have any in depth technical knowledge as they will only have to be able to administrate the account- in essence it divorces them from the physical server. Since this is a paid service, you will of course have access to customer support and contracts as well as official guarantees of space and uptime performance.
There are also some downsides that should be noted, though, primarily that you are actually sharing the server's resources so this can adversely affect some performance. There is a physical limit to the amount of memory that the server will have, as well as limits that govern the CPU and disk space that will be available, so if there is already a heavy load on the server from the other customers (perhaps because the host has oversold the space!) you will have performance issues with your site. This means that things entirely out of your control can slow down your site.
You may also notice if you are technically inclined that shared hosting rarely allows for use of advanced and more powerful options that high end solutions give you. There also are some security risks that can be inherent in this type of set up because you are on the same physical server as other people, so if they are doing things that can compromise security they could also end up compromising your security as well.
In the end, many people will find shared hosting to be a great solution for their needs. The only people that will not find many advantages from using this form of service are those that want lots of control over their own server and its contents.