Why do you think George asks Lennie to familiarize himself with the location of the clearing where they spend the night?
If the reader finishes the novella, Of Mice and Men by John Steinbeck, and then returns to the first chapter and rereads it, he/she will apprehend that almost everything that happens in the plot is laid out in this first chapter. This pattern of Steinbeck's metaphorically reflects the line of Robert Burns's poem, "To a Mouse," whose line the novella's title has borrowed:
The best laid scemes of Mice and Men, gang aft agley
[The best laid plans of Mice and Men often go awry]
George lays the best plans he can--as does the author, Steinbeck--, conditioning Lennie to return to the brush by the river if anything "bad" happens, but like the mouse who builds its nest in the field that is mowed in Burns's poem, the "nest" of George and Lennie's dream is destroyed in the death of the figurative "mouse" in Lennie's bear paws, Curley's wife, that Lennie inadvertently, like the mower, kills. With the dream of George and Lennie destroyed, there is no real safe "nest" to return to: indeed, "the best laid plans go awry."
Like many people of that time, the Great Depression, George is one of those people who would always have a backup plan, a plan for an emergency. Since he hangs out with Lennie, he's more likely to have an emergency plan than ever. Lennie, as he says, "Gets me in hot water all the time". Later in the story, Steinbeck also tells us about an incident which happens "up in Weed", where Lennie gets hold of a red dress and won't let go - trouble which forced them to run away from men with dogs, hunting them. So George knows from painful experience that he might not always be nearby to help Lennie out, so he plans for a rendezvous spot in case he gets into trouble again.
To answer this question, just look at the little bit after the part where George tells Lennie to remember the place where they are. George tells Lennie that he should come back to this place and hide in the bushes if he gets himself into some sort of trouble.
When George tells Lennie this, he mentions that Lennie has gotten himself into trouble before (he talks about this earlier in the chapter as well). So George kind of expects that something bad will happen again. If it does, he wants Lennie to go somewhere safe until George can come for him.