Why is it essential that both the plasmid and the donor DNA be exposed to the same restriction enzyme?

This question refers to genetic engineering. The basic idea is that cells are induced to make some protein or other substance that they normally do not. Plasmids are used to carry the new DNA into the cells. Restriction enzymes are those that "cut" the cells DNA, so that the new plasmid DNA can be inserted.  Restriction enzymes "cut" DNA at various points--if the plasmid and donor DNA are exposed to different restriction enzymes, they will be "cut" at points that will not allow the plasmid DNA to be inserted in a way that allows the foreign protein to be made.