Inevitably we ask what the name Mamre contributes to the story.
Is Mamre a place or a person?
One view is that it is a township, possibly identical with Hebron. Josephus says that in his time people used to point out the actual oak tree at Mamre where Abraham found shelter from the sun.
Another view is that Mamre was a chieftain – possibly deriving his name from the district where he lived – involved in Abraham’s recapture of the spoils taken by Kedorla’omer (Gen. 14).
Rashi followed the second view and said that Abraham consulted Mamre about whether he should agree to circumcision. Mamre said “yes”, though we wonder why the question was necessary once God had already spoken.
The explanation could be that (whether or not one needs the advice) consulting people shows them respect and gives them a sense of involvement.
Even God asks advice, regardless of whether He really needs it: he asks the angels whether to create man, and the answers are not all positive.
Just as God does not want anyone to accuse Him of being high-handed and peremptory, so every one of us needs to feel and show that we have others on our side.