ASIC Prototyping: Is it design or verification?

Why does the ASIC developer need ASIC prototyping? Is it to learn how their existing design will function with a new interface -- say to DDR3 memory? Is it to completely verify their ASIC design prior to tapeout?

The wiktionary provides one definition of a prototype:

An early sample or model built to test a concept or process

The idea that the prototype is early suggests that ASIC prototyping is part of the design activity when the design space is being explored.

Yet, if a prototype is built primarily for testing (verification) then how will you track coverage? I can see the manager, newly trained in understanding the significance of coverage metrics asking, "Did you test everything in the ASIC prototype?" Simulation seems to rule in this area but how long will
the simulation take for a big SOC?

(My first ASIC prototyping experience was with a wire-wrapped mass of TTL dips on 4 very large prototyping cards soldered together at the edges and held in a very large oak frame. Decidedly unsophisticated versus today's standards but effective.)

Considering the complex environment that ASICs operate in perhaps the prototype serves the purpose of an at-speed bus functional model to be used with the surrounding system hardware. For example, such a model could provide meaningful interaction with PCIe driver firmware on the host prior to the availability of
the actual device.

What ever the underlying goal I'm sure that ASIC prototyping is necessary. It just isn't clear to me how it is defined. When I find a formal definition that works I'll publish it at ASICPROTOTYPING.COM.