George is afraid that if his new boss finds out about Lennie's "limitations," they will not be taken on at the farm and will have to move on to someplace else. So he tries to cover for him by answering in his place.
Remember that George and Lennie were "on the run" from a previous town called Weed. Lennie had torn a lady's blouse and was accused of sexual harassement. Consider the precarity of the migrant workers of the time. There was no insurance or compensation when they were out of work and these drifters lived one day at the time, moving from one farm to another according to the season.
As they had to leave Weed on the spot (and probably without getting their pay), it was important for George and Lennie to find work again as soon as possible just "to get by."