You can nationalize this corporation (Fearless Blue)

what does it do differently than it would if i didn't do it?

A bid, hostile or otherwise, cannot be placed on nationalised corps.

That is the only difference between a state corporation and a nationalised one.

Why is there a limit imposed on the maximum number of nationalised corps? Im guessing its to ensure that there are enough corps ready to be privatised that the market does not cease to function. It also seems to prevent individuals from running an entirely state-owned country.

Is there another reason?

Bid on mine without permission and i'll declare. There's one differance.

It would appear, then, that dedicated economy players who have not the resources for war are the least able to be state-owned only countries.