Why are there angels?

I understand that the translation of the Hebrew word מלאך means "messenger" and an angel is "G-d's messenger."

We see angels mentioned in various roles. Frequently, angels appear in dreams. Sometimes, they are assigned tasks such as the three angels that appeared before Avraham.

If G-d is present everywhere, and he is able to do everything, even doing many things simultaneously ("multitasking"), then, why does G-d need to send angels or messengers? Why doesn't G-d just do the job, himself?