Three-tiered architecture Installed as Two-tier

Sometimes the application and database layers are combined into one server and hence three-tier system becomes two-tier.

From an installation point of view the database and application server are treated as two machines.   You install database software and then you install applications software on the same machine.

This is way you can achieve simple-to-administer system however you loose on scalability.  

When the server runs out of resouces, it has to be upgraded or replaced as a whole. You cannot simply add another application server machine or database sever machine to the existing system. 

It is for scalability, the three-tiered architecture is so popular.