Halliday & Matthiessen (2014: 93, 95):
In a thematic equative, all the elements of the clause are organised into two constituents; these two are then linked by a relationship of identity, a kind of ‘equals sign’, expressed by some form of the verb be. …
A thematic equative (… a ‘pseudo-cleft sentence’ …) is an identifying clause which has a thematic nominalisation in it. Its function is to express the Theme–Rheme structure in such a way as to allow for the Theme to consist of any subset of the elements of the clause. This is the explanation for the evolution of clauses of this type: they have evolved, in English, as a thematic resource, enabling the message to be structured in whatever way the speaker or writer wants.