This just makes reputation points into a popularity-contest, simply suppressing dissension by opinions rather than facts.
The purpose of down-votes is explained in our Help Centre
Voting down, also known as "casting downvotes", is how the community indicates which questions and answers are least useful.
It is part of the SE 'gamification' model (searching for
gamification on Meta:SE will give you more background about this model).
For questions, the reasons for down-votes are indicated by the tooltip on the down-vote button:
"This question does not show any research effort; it is unclear or not useful"
(place your cursor over the down-vote button to see the tool-tip)
Similarly, for answers, the tool-tip states:
This answer is not useful.
Of course, the 'usefulness' of an answer is almost always going to be subjective! As a rule, you can make answers more 'useful' by ensuring that all assertions are supported by citations from reputable sources.
Our Help centre contains a page, How do I write a good answer?, which you might also find helpful.
More specific to this site, you might find the information contained in the threads:
here on meta to be helpful.