Why does it matter that DNA would be in (right-handed) B-form than in A form?
It is called B form due to the twists in the coil being of a particular distance, longer than the A-form. This has implications for the recognition of which proteins may have been acting as transcription factors (their size has to be such that they’d fit into the groove and therefore properly react with the DNA). In fact, the researchers who first crystalized dsDNA had dehydrated it a little too much and thus compressed the twists, reporting dsDNA was in A form at first. As a result of this error, certain proteins were initially disregarded as potential transcription factors when they in fact were such.