Sanji's main "weapon" is his legs. As we know with Queen's brachiosnake attacks, Advanced CoA, and Advanced CoC shown in this arc, Armament Haki itself won't be enough to make Sanji top tier. There are attacks that goes through defense in top tier fights.
Hence the Exoskeleton comes in, his legs/body itself are really durable. This allows him to fight a top tier battle and damages highly durable people.
Else, a few attacks like Brachiosnake/AdCoA/AdCoC will broke Sanji and render him useless. We can't have that. See Vergo situation.
Else, when he kicks ultra durable stuff, it backfires. See Kuma situation.