why is it useful to a cell for the two polynucleotide strand

Solution

It is useful for a cell when hydrogen bonds link the strands of DNA as it stabilises the molecule. One hydrogen bond is weak but collectively, many hydrogen bonds forms a very strong force to keep the DNA strands linked together. Also, the polar groups which are the other groups in the base rings will be able to form hydrogen bonds externally with the surrounding water to further increase the stability of the molecule.