Flexibility - By separating the business logic of an application from its presentation logic, a 3-Tier architecture makes the application much more flexible to changes.

Maintainability - Changes to the components in one layer should have no effect on any others layers. Also, if different layers require different skills (such as HTML/CSS is the presentation layer, PHP/Java in the business layer, SQL in the data access layer) then these can be managed by independent teams with skills in those specific areas.

Reusability - Separating the application into multiple layers makes it easier to implement re-usable components. A single component in the business layer, for example, may be accessed by multiple components in the presentation layer, or even by several different presentation layers (such as desktop and the web) at the same time.

Scalability - A 3-Tier architecture allows distribution of application components across multiple servers thus making the system much more scalable.

Reliability - A 3-Tier architecture, if deployed on multiple servers, makes it easier to increase reliability of a system by implementing multiple levels of redundancy.