Aero Catches

When I first got my Clio, the cable was frayed, the catch was falling apart, so it seemed sensible to fit bonnet pins. No more worries/problems in that department.
 
Why does one need aero catches?

Because I want them instead of a busted cable I currently have.

Has anyone got any tips to fitting these? I fitted a set about 8/10 year ago I think and cant really remember how they went.