Why does DNA do this?

The cell must distinguish between mature RNAs that encode its proteins, and other RNAs produced either as byproducts of maturation (e.g. excised exons) or by errors (e.g. wrongly spliced mRNAs). Mature mRNAs are selectively exported from the nucleus into the cytoplasm, where they are translated into protein. Details of the selection mechanism are incomplete, but it is based on proteins from each processing step remaining bound to the RNA. These proteins include cap-binding proteins, poly(A)-binding proteins, and proteins added to completed splices. These signs of maturation are recognized by proteins responsible for export through the nuclear pores. This mechanism allows mature mRNAs to leave the nucleus, whereas other RNAs are degraded and recycled.