A three tier design separates the core and distribution layers and are recommended when there are more than two pairs of distribution switches required. This creates a core, distribution, and access layer in the network.
Multiple pairs of distribution switches are usually required for a few reasons.
One can be that there is a network that compromises of multiple buildings, and each building needs its own distribution layer.
Similar to the first reason, an additional cause can be the dispersion of access layer switches across many buildings require too many cable runs to a single collapse core.
Another reason can be that the density of WAN routers, Internet edge devices, data centre servers and network services grow to a point where network performance can suffer.
When there are many distribution layers that need to be connected together, it can become a requirement to use a core layer.