Halliday & Matthiessen (2014: 144):
The Finite element, as its name implies, has the function of making the proposition finite. That is to say, it circumscribes it; it brings the proposition down to earth, so that it is something that can be argued about. A good way to make something arguable is to give it a point of reference in the here and now; and this is what the Finite does. It relates the proposition to its context in the speech event.
This can be done in one of two ways. One is by reference to the time of speaking; the other is by reference to the judgement of the speaker. An example of the first is was in an old man was crossing the road; of the second, can’t in it can’t be true. In grammatical terms, the first is primary tense, the second is modality.