Halliday & Matthiessen (2014: 289):
The verb be can be used in all categories set out in Section 5.4.2 although have is the unmarked verb in ‘attributive’ clauses of possession (in standard English we say Emily has a piano rather than with Emily is a piano). The variants of ‘possessive’ and ‘circumstantial’ clauses with be (and have) are analogous to ‘intensive’ clauses. Thus Emily has a piano can be interpreted as ‘Emily is a member of the class of piano–owners’ and the meeting is on Friday as ‘the meeting is a member of the class of the class of events on Friday’. Similarly, the piano is Emily’s can be interpreted as ‘the piano is identified as the one belonging to Emily’ and Friday is the best time as ‘Friday is identified as the best time’.