As you say, Security is an important part of this model as well as HA and functional separation. In much the same way as we can logically collapse networking layers (core, agg, access), you could do the same with server tiers.
The multi-tier data center model is dominated by HTTP-based applications in a multi-tier approach. The multi-tier approach includes web, application, and database tiers of servers. Today, most web-based applications are built as multi-tier applications. The multi-tier model uses software that runs as separate processes on the same machine using interprocess communication (IPC), or on different machines with communications over the network. Typically, the following three tiers are used: