The advantages of this multi-tiered model include the following:
Encapsulation of business logic in a shared middle tier. Different client applications all access the same middle tier. This allows you to avoid the redundancy (and maintenance cost) of duplicating your business rules for each separate client application.
Thin client applications. Your client applications can be written to make a small footprint by delegating more of the processing to middle tiers. Not only are client applications smaller, but they are easier to deploy because they don’t need to worry about installing, configuring, and maintaining the database connectivity software (such as the Borland Database Engine and the database server’s client-side software). Thin client applications can be distributed over the Internet for additional flexibility.
Distributed data processing. Distributing the work of an application over several machines can improve performance because of load balancing, and allow redundant systems to take over when a server goes down.
Increased opportunity for security. You can isolate sensitive functionality into tiers that have different access restrictions. This provides flexible and configurable levels of security. Middle tiers can limit the entry points to sensitive material, allowing you to control access more easily. If you are using HTTP or COM+, you can take advantage of the security models they support.