 Each program needs separation.  Better computer security by not installing multiple programs on one system.  Most out-of-the-box software assumes it is the only program running on a computer.  Thus, each piece of software is installed on a separate computer.

 Each computer is hardly using its resources.  Each computer uses power ($1k/year) and rack space/cooling.  The more computers you have the more hardware there is to possibly fail.

 One computer pretends to be multiple computers.

 Saves on power and space  Easy to make use of the cloud  Simplifies many configuration tasks  Eliminates many sources of hardware failure  Saves money

 More computers on one box mean more can go wrong at once  Learning curve  Initial investment to get to the cost savings