Is it necessary to have armoural put on a new car to prevent rust?
Nope, all cars have pretty extensive corrosion protection and warranties, and treatments can sometimes cause problems.
No that Armourall is only needed for ADP (Additional Dealer Profit)
While I would guess there are some good thing to say about the stuff (if you like shinny tyres for example) but there are bad things about it as well. Actually they make several products.
Modern cars come with rust prevention built in by design. The choice of materials, drains various coatings and other designed in features do far more than anything that can be added later. Many of the after market products will actually cause more rust, but defeating the built in ant-rust design.