why is necessary a shield with 74LV245 chips ?

Why is necessary a shield with 74LV245 chips to connect the Arduino Due to the lcd module ?

Can I connect directly the Arduino Due module to LCD ?

You don't need 74LV245 chips to connect the Arduino Due. The Due has 3.3V logic.

I suspect that your BuyDisplay Adapter shield has them anyway. They will do no harm.
Most importantly, the Display + Adapter + Due sandwich fits together neatly.

You can hand-wire a Protoshield to perform the Adapter function.
There are a lot of wires to solder. Your time costs more than buying the Adapter.

David.