It is used to emphasize the Subject (model 1), the Object or the Adverbial modifier (model 2). The word order in such sentences is the following:
|It + to be + (emphasized part of the sentence) +  ||  who/which + predicate + the rest of the sentence|
|  that + subject + predicate + the rest of the sentence|
Model 1 is used to emphasize the Subject, model 2 – the Object and the Adverbial modifier.
A special type of this structure is the model with not … until.
It was not until the dawn that we returned home.
Мы вернулись домой только на рассвете.
Double negation gives affirmation. This model is used to show that the action took place late.