People can become deeply socially programmed, indoctrinated, to find certain classes of people attractive, or more famously, unattractive, e.g., racism. Why?
At first glance, susceptibility to social programming affecting mating decisions seems evolutionarily disadvantageous. Such an instinct could easily be subverted to steer you away from mates who might be objectively better for the ultimate goal of propagating your DNA to future generations; perhaps a mate that is more fit for the environment.
There is probably a reasonable explanation based around tribe dynamics, with the instinct actually driving you to do an unconscious, or not consciously controllable, social calculation to assess how successful your offspring will be based on the social position of a potential mate. These social factors are just as much a part of the environment for a fitness calculation as (say) the climate. Nevertheless, it still remains mysterious that a not consciously controllable calculation can override conscious calculations of fitness of a mate.
What is love? Thus, the biological answer could be quite complicated, encompassing these neurological and biochemical mechanisms of mate selection.
Is this instinct still useful in modern society? On one hand, I suspect it is getting subverted (as mentioned above) a lot, perhaps the root of massive social conflicts. On the other hand, society still exists, and assessing potential mates in the context of society is still likely helpful in determining the success of your offspring.
Understanding this instinct may be key to solving society's massive social conflicts.