Wooden fence, wire, staples, why?

[thepurpleblob]

I am about to replace a wooden fence in my garden and am contemplating a style.

Anyway... I've noticed that lots of wooden fences made with vertical boards have a wire running a few inches from the top stapled to each board.

What's it for and why do I want one?

 

[foxhole]

Adds strength, prevents boards being just pulled out to gain access.

 

[imamartian]

what's a purple blob?

 

[thepurpleblob]

too obvious really

Thanks