These tasks are intended to construct background knowledge. The teacher becomes a bridge builder between what students already know about a concept - schemata - and what they need to know in order to understand a particular text, that is, the interaction between those schemata and the input coming from the text.
Pre-reading tasks are intended to prepare the learners for a reading selection, or to give them the first steps in order to develop skills in anticipation and prediction for the reading, activating background knowledge so they could later interact with the text. With these tasks, teachers give students meaningful pieces of information that they would encounter in the reading.