Managed Hosting provides  :

Bandwidth : Managed hosting providers can often supply bandwidth more cheaply than you could yourself. By hosting many clients, they achieve economies of scale which could never be achieved by a solo operator.

Backup power : An UPS (uninterruptible power supply) is an essential for any serious web-applications. If the power goes down, mission-critical web operations should not go down with it. A managed host can provide this – again, usually at lower costs than you could yourself.

Security : Servers need to be constantly monitored to prevent unauthorized access, denial of service attacks, the spread of viruses, etc. Installing the necessary firewalls and monitoring equipment can be a costly exercise, which, again, your managed host can help prevent.

Staff : Hiring the systems administrators with the expertise to keep your servers and networks up and running can be costly – managed hosts can provide all of this for a set monthly fee.

Redundancy : You will need back up capacity in case there is a failure on your server. The best way to achieve this reliability is to use redundant servers. Then, if one server fails, another can take over. (This use of multiple servers to ensure reliability is often referred to as “clustering” in order to achieve “load balancing”.) Again, your hosting provider can often provide this redundancy at a lower cost than you could yourself.

Data Backup : If there is a systems, network or power failure, the last thing you want is to lose your data. You will need equipment in place for regular, daily back up of your data. A managed host can provide this.