Halliday & Matthiessen (1999: 399):
For instance, an identifying clause may be used to represent a non-identifying one, thereby providing an alternative construal of that other clause as a configuration of Identified + Process + Identifier. Thus, the clause you want this may be reconfigured as an identifying clause by nominalising 'the thing that you want', what you want, and identifying it with this, either as what you want is this or as this is what you want … The two versions of the clause are related in Figure 9-14.
The motivation behind the identifying metaphor is actually textual: the alternative configuration in the identifying clause constitutes a textual alternative for distributing information in the clause, in which the message is structured as an equation between two terms. This system is known as THEME IDENTIFICATION.