When you have heavy movements of materials, goods deliveries then it is better to track all those outgoing deliveries, material for repairing to verify what is issued and what is delivery. In some countries it is also used as a proof document for tax exemptions and computations too.
Gatepass is the document prepared by the Warehouse users and attached with delivery order to instruct the gate kipper to allow deliveries or transfer of materials. Based on the types of gate pass system decide that it is returnable material or not. Usually when we sale it is non-returnable materials.
Go through below vides how gate pass is working in OpenERP.