So that there is space for molten metal to expand.

So that the pattern can be removed easily without imposing too much friction which may drag some sand along, causing the intended shape to be affected.

So that a core can be inserted into the mould easily, in order to create a hollow feature within the component that is being cast.

So that the pattern will be symmetrical.

So that the molten metal can solidify evenly where it comes into contact with sand within the mould.