One use case would be one that came up a while back here on HN -- The person had a large binary blob that they needed to load up every time their Lambda function ran. If they could mount EFS from Lambda, they could put the large binary blob there.

Depending on how they wrote their code, if the Lambda could seek to the right part of the file then this would work, but if it has to load the entire file into RAM anyway, then it wouldn't gain them much except perhaps a slightly easier way to get the file into RAM.