It's good for all of us
Rating what you watch isn't only important to improve your own recommended viewing, it can make a big difference for your fellow Netflix subscribers. That's because the service's algorithm takes into account a number of factors beyond our own control, including the combined ratings of all Netflix members who have similar tastes.
In short, Netflix lumps us into groups based on our similar viewing habits, and uses the collective ratings from us all to better predict how well we'll like something. This is why, for example, the percentage rating you see for 13 Reasons Why may be dramatically different from the one your teenage cousin sees. You have vastly different tastes, and so your rating is based on how people who tend to watch the same stuff as you rated 13 Reasons Why, versus how people who tend to watch what your cousin does rated it. Therefore, the more we're all rating what we watch, the more accurate these percentages will become for everyone.