... as it's also done for every other type of turbofan or turboprop airliner - not only Airbuses (even though I think it is only referred to as "flex" on Airbus and Fokkers - but I might be wrong).
The assumed temperature method allows for reduced thrust take-offs, therefore preserving the life of the aircraft's engines. The use of laptops, however, is company-specific (it is more commonly done by reference to performance tables).
Cheers
