why does the player taking free kicks/corners,raise both arms?

pre-arranged signal with teammates to show how he intends to play the ball in (eg, near post, far post, etc)

It's usually some kind of signal to tell his team mates where he's going to put the ball in the are. One arm can mean near post, two arms far post etc.

To show that he's about to take it so players can start making runs in the box.

Let people get ready and people puts the hand up in the box so he can aim for him

Lol

It just means its a high ball for the tall/big men on your team.

To get ready

Smelly armpits lol