The purpose of the bulge is because the inlet manifold of the F7P/R engines(16v/Willy) would foul a normal Clio Mk1 bonnet.
The vent cools the exhaust manifold, which is at the back of the engine bay - next to the bulkhead. I dont know about the 172, but most cars seem to have their exhaust manifold at the front of the engine bay - where they can get cold air more easily. Hence the bonnet vent on a 16v/Willy.
Looks so distinctive though. Matches the wide arches too. I wish theyd beefed up the rear arches on the 172 as well - the 16v/Willy share only the roof/doors/boot with a normal Clio. Mind you, the 172 isnt exactly ugly!