The grid is used to aerate food waste in a non-tight bag, for example a paper bag, and in this way ensure good ventilation.
The food waste will dry and as a consequence leakage and bad odor will diminish, possibly go away completely. The grid is included if you buy a Recyclers or a Basic sets. If your municipality don’t want you to use these kinds of non-tight bag you can simply skip the use of the grid.