Yea, you have the right idea. It is zipped together which allows the important part (the bases) to be protected from things in the cell. Also, because DNA is an information carrier, it's important that information is not lost or changed. Loss of information or change will be the result of a mutation. But luckily for the cell - it has a backup. Since the DNA is double stranded - we know the second strand by knowing the sequence on the first, and vice versa. So even if a mutation occurs on one strand, doesn't necissarily mean that the other strand has been effected. The mutated strand, when replicated, will produce mutated dsDNA (double stranded DNA) but the un-mutated strand will give rise to wild-type dsDNA. This idea that, even if a mutation occurs it doesn't mean that information is lost, is important and possible because both strands have a sequence dependent on the strand they were synthesised from.
Final year genetics