I suppose it's because foodstuffs can absorb harmful things from certain materials, then you'd ingest them without realising it.. By having the symbol, you can be reassured that this won't happen with whatever item you're using for storing, preparing, cooking, or eating from.
As an example, cheeses and fats will absorb the elastomers from cling film, so the advice is to not have the cling film in direct contact with these products for any length of time. (And heat increases the rate of absorbtion).