Can one rewrite this sentence "They are busy collecting food for the winter" like this : "For the winter, they are busy collecting food."
Similarly the following is strange: "For charity, we are collecting money."
However, if you're making a comparison, it sometimes works:
"For Mary, I'm buying a dress, and for Peter, I'm getting a new tie."
"For the summer she went to Brighton, then for autumn she stayed in London." (Note this means 'during', not "in preparation for".)