原句用 as 正确，说明了 bread and butter 的身份，不可以改为 for。
as 和 for 的区别，请仔细体会：
At that time, many farmers could only afford bread and butter as their meal. （身份）
At that time, many farmers could only afford bread and butter for their breakfast. （用途或目的）
meal 表示“吃的食物”。
breakfast 表示“吃饭时间和性质”，本身不是食物。
如果仍然不解，再看这两句：
This room is used as a storage room. （身份）
This room is used for storage. （用途或目的）
