At the time, it was assumed that genes and traits had a mostly one-to-one correspondence. Thanks to the human genome project we now know that it’s more often a many-to-many correspondence, which makes figuring out the relationships enormously more complex. But mapping the genome was still a critical step.

Edit: The analogous situation in neurology would be the correspondence between brain regions and cognitive functions—in the last decade or two we've found out that most functions involve many separate brain regions networked together in different patterns for different functions. Mapping this “connectome” is the equivalent of mapping the genome—but this Harvard/Google neuron-mapping project is at a much lower level, more akin to studying the physical structure of chromosomes.