Relocating the business logic onto a middle-tier server means that the same business services can be accessed from many different types of clients. These clients can potentially use one of multiple, interchangeable middle-tier servers that offer the same services. These servers can be upgraded, or more servers added, as the application scales
All applications contend for certain scarce resources, such as database connections. By concentrating, these resources in the middle tier, caching and recycling strategies can ensure that many clients share these resources optimally between them.
By delivering business services from interchangeable servers as described above, a server failure is no longer catastrophic. Clients of the failed server can be redirected to another server providing the same services.