Sometimes reality is shown to be so grim that retreating into imaginary worlds in our imagination is the only way in which humans can comfort themselves. This is a strategy that Offred uses in Chapter Seven, when she reflects on how her nights when she is by herself are the only time that she truly has to herself. She uses this time to retreat to places in her imagination in the past, remembering a memory of when she was at college with Moira and then a memory of her childhood with her mother before reflecting on what happened to her own daughter and how she was taken away. Imagination, as Offred makes clear, isn't just a luxury or some kind of distraction, it is actually an essential survival skill, as this following quote from Chapter Seven makes clear:
I would like to believe that this is a story I'm telling. I need to believe it. I must believe it. Those who can believe that such stories are only stories have a better chance.
Imagination then is shown to be an essential survival mechanism in order to help Offred cope with the grim reality of her present situation. Imagining past times when she was not the trapped, restricted individual that she is in the present gives her the strength to carry on and, as the quote suggests, increases her chance of survival.