The Preliminary Phase is required every time there is a new business directive for EA. You could say it establishes the parameters for a successful iteration of the ADM. This short video gives a quick overview of this phase.
The main objectives of the Preliminary Phase are to determine and establish the Architecture Capability desired by the organization.
A key part of this is to define what needs to be done, and how it will be carried out. For example, the main output is a Request for Architecture Work that outlines requirements, and deciding what organizational context, structures, tools or architecture frameworks need will be needed to support this work.
In this phase TOGAF is tailored to meet the needs of the coming iteration of ADM. We define underlying principles, assess the ability of the enterprise architecture (EA) and business to make the required changes, and integrate TOGAF with other management frameworks.
There are steps in this phase to scope the enterprise organizations impacted from the proposed changes, to confirm that the right governance and support frameworks are in place, to define and establish the EA team and organization, to identify and establish architecture principles, to tailor TOGAF and any other frameworks, and to implement tools.
At the end of this phase, the EA team should be ready to follow an iteration of the ADM cycle. This is partly why the Preliminary Phase is shown at the top of the ADM diagram, and outside of the main cycle of Phases A to H.