The weakness of hydrogen bonds between the bases of DNA allows for the easy separation of the two DNA strands during replication of DNA.
In replication, an initiator protein unwinds a short length of the DNA double helix. Next, an enzyme known as helicase attaches itself to the unwound helix and begins to break the hydrogen bonds between the base pairs of the two DNA strands thereby pulling them apart to form a Y shape called a replication fork.
The relative weakness of hydrogen bonds between the base pairs make it fairly simple for helicase enzyme to beak them apart and thus the process is done very rapidly.