Basically, as air in the house gets warm, it rises. Without insulation, the warm air will be lost via the roof (and any other non-insulated surface), as cold air enters causing temperatures to drop. Such erratic temperature changes are undesirable and so roof and attic insulation is vital to keep your house warm.
That’s why strategic sections of house are insulated. Walls, floors, crawlspaces, basement, attic or any other spot that can allow air leakage. Of these, the non-insulated flat roof or attic contributes to over 50% heat loss for the entire house.