Transcription of gene into RNA ,and translation into amino acids ,query ?

A couple of points:

For any single gene, only one of the two strands is transcribed. However, move up or down the molecule from that gene, and the other strand is transcribed. So, both strands are used to code for proteins, but at any single gene, the other strand is redundant.

The double-stranded molecule allows cells to correct many mutations. During replication when most mutations occur, mechanisms exist to distinguish the "older" strand from the newer. When errors are randomly introduced into the new strand, the older is used as a template to correct the new mutations.

The double stranded molecule allows for very efficient replication to occur. If DNA were single stranded, you would effectively have to make two copies in order to replicate the single strand--first you would make the complementary strand, and then copy that to make a new single strand. 

What hcbiochem said, and:  double-stranded DNA doesn't tangle into a big knotted mess.

I have kite string.  Some fibers tangle up.  Some don't.  Think of double-stranded DNA like the "don't tangle" stuff.