Why is it important that the nitrogenous bases (A, T, G and C) are weakly bonded together by hydrogen bonds?

When the cell divides, it "unzips" the DNA to copy it. Therefore, the bonds must be weak. However if there were no bonds, the DNA would simply fall apart.

During cell division, the DNA must be replicated, since the daughter cells need a complete set of DNA. To create TWO DNA, it must replicate. To replicate, DNA unzips, the bases pair up with a different base, and the backbone bonds.