They are just trying to express themselves which they feel they can't always do in words so they also gesture to labour their point. It often becomes a habit as Gordon Ramsey is case and point as he feels it hit home his point but just gets annoying after a while.

Gesticulation is a very powerful tool in adding emphasis to the spoken word. Some people have a natural ability to deliver appropriate gesticulation to embelish their speech whilst others have to work hard at trying to master it. Dan Corbett, a BBC weather presenter is famous for his gesticulation accompanying his delivery of the forecast.
http://www.youtube.com/watch?v=TZly6LCDscM

i think it is to try to elevate thier voices by trying to fly

I did not realise how much I done this till I caught people laughing when I once gave directions to someone over the phone and I done right hand signal when telling them to turn right, action for up hill, round roundabout etc. Was told my directions and actions were very good if only caller could see them!!!

Because she's Portuguese (my wife that is) and they're nearly as bad as the Italians for this.

Old joke, I know:

Q How do you get an Italian to shut up?
A Make them sit on their hands!

With most people it's learned behaviour. They learnt it from their parents or whoever looked after them most during childhood. Presumably some time in the past a lot of people had difficulty explaining what they meant when only using words. I would imagine that the many different dialects in Italy, for example, would have had something to do with it.

How else would you demonstrate how big fish you caught?