What is the importance of DEFENSE ?

the ivs have influence on the max cp, so obviously they change some of the base stats. I'm not sure how this works, but I think the defense iv put a % limit on the defense stat of the pokemon, same way for atk and HP. If you have 2 pokemon of a same specie in the same evolution ( 2 oddish, 2 magikarp...) in the same level, one with 15 HP and one with 0 you will see the difference on the poke hp, it must be the same thing to atk end def, but you can't see.

not really, Dragonite already has really high defence.

Against quick moves it'll make basically no difference due to rounding. Against higher damage quick moves, it might be the difference between like 3-5 damage.

https://pokemongo.gamepress.gg/damage-mechanics

Using an example, say you're hit by Hyper Beam on Snorlax of equal level to your Dragonite.

damage = 0.5 x 120 x 190 / 201 x 1.25 = 70.89 which gets floored +1 to 71.

a dragonite with 15 defence IV has 216 defence so it'd take 66 damage instead. seems like a lot of damage but most of your damage taken comes from quick moves.

for weaker moves it will rarely make a difference. take a water gun vaporeon.

damage = 0.5 x 6 x 205 / 201 x 1.25 x 0.8 = 2.83 which gets floored +1 to 3.

with 15 defence IV that damage is "reduced" to 2.64 which still results in 3 HP lost, making no difference.