Why Allah is self introducing in Quran by using the word "we" instead of using "I"?

Why Allah is introducing him self by using the word we instead of using I? Here I'm adding two Ayaths from surath yaseen 8. Verily! We have put on their necks iron collars reaching to chins, so that their heads are forced up. 9. And We have put a barrier before them, and a barrier behind them, and We have covered them up, so that they cannot see.