MRNA is needed in the cells because it directs the synthesis of proteins which is found in the cytoplasm. It is a molecule that carries the message contained within DNA to the ribosome. The Proteins are assembled on the ribosomes using the mRNA nucleotide sequence as a guide.
ATP, NADPH, and oxygen are formed products of the light-dependent portion. The independent part would produce things like glucose or lowered amounts of energy and NADP, much lower.