A basic rule of thumb is any time you add a tier, scalability, availability,extensibility, manageability, maintainability, reliability, security, and performance improve. There is, of course, a law of diminishing returns that states that at some point, more tiers will degrade performance, availability, and reliability as there are far more points of failure.
Architecture is a set of structuring principles that enables a system to be comprised of a set of simpler systems, each with its own local context that is independent of but not inconsistent with the context of the larger system as a whole.
Scalability is the ability to support the required quality of service as the system load increases without changing the system.
Reliability ensures the integrity and consistency of the application and all of its transactions.
Availability ensures that a service/resource is always accessible.
Extensibility is the ability to add additional functionality or modify existing functionality without impacting the existing system
Maintainability is the ability to correct flows in the existing functionality without impacting other components of the system.
Manageability is the ability to manage the system to ensure the continued health of a system with respect to scalability, reliability,availability, performance, and security.
Security is the ability to ensure that the system cannot be comprised.