If I recall correctly, George tells Lennie to remember the spot so that he will know where to meet should he get into any trouble. George wants Lennie to have a safe place to hide until he can help him.
Ironically, it is in this safe place that Lennie meets his end, and George is the one who takes Lennie's life. He does this to help protect his friend from an even worse fate--being caught by Curley and the lynch mob.
George has Lennie remember this place because if anything happens in the future at their new job, Lennie can come back here and George can meet him there for their getaway. Remember both men are fleeing what happened in Weed, where Lennie frightened a little girl and they had to leave. So George is just thinking ahead in case anything goes wrong. This is an important bit of foreshadowing that is important later in the novel.