Why must "Auzubillahi Min ash Shaitaan ir Rajeem" be our first words we say before begining to read the Quraan, yes its one of the ettiquettes for when one wants to read the Quraan. (Meaning: "I seek Allah's refuge against Satan, the rejected")
The very first reason is, because Allah has commanded this in the Quraan itself,
"And when you recite the Quaan, seek refuge with ALLAH from Satan, the rejected" [16: 98]
And Satan is an enemy to us, because we are friends to Allah (the God), or are meant to be.
And the Quraan is the way to God. Satan would never want us to reach God, and get His pleasure, because he himself was rejected by God due to his own arrogance.
So he comes in our way, as we start to get guidance through Allah's words. He starts putting the opposite thoughts in our mind, so that we may not understand it correctly, get confused or even close it and place it aside.
Actually through this we are tested. NOT that Satan does that by having some power out of God's control. Ofcourse not, he is let to do all this by Allah. So for us it is a constant struggle. Another 'Jihad'.
"I seek Allah's refuge against Satan, the rejected"
And these words are a weapon. But remember our enemy is all time alert and very determined, he would keep attacking as he has vowed against us. That means we got to be equally resolute.
Even with you holding your weapon, he would be setting on you. Now you put your trust in Allah, because he cant harm unless Allah wills it. As Allah says, 'And upon All the believers should trust" [58.10].
Now be sure that you are protected by Allah, because you asked for it.
"And if an evil suggestion of Satan assail you, then seek refuge in ALLAH; surely, HE is All-Hearing, All-Knowing.
As to those who are righteous, when an evil suggestion from Satan assails them, they remember ALLAH and behold ! they begin to see aright. "[7:200-201]