Why do you think it is important that the sugar - phosphate backbone of dna is held together by covalent bonds, and the cross - bridges between the two strands are held together by hydrogen bonds?

As covalent bonds are strong it makes the sugar phosphate backbone a stable molecule which will HV less chances for a mutation while the huydrogen bonds exist for the ease of DNA replication as they are strong in a way but weak for the replication to take place easily.