Eugene Volokh presents an example of why interpersonal relationships are important. The details are related to academics, law school, developing relationships with professors, and getting letters of reference, but the principles are universally applicable. Who will a professor think more highly of: a student who gets 100% on tests but never talks to him, or a student who gets 90% on tests but asks questions, sticks around after class, and goes to office hours? Ultimately, which student is more likely to be successful?
Grades are important, but when dealing with humans you must always remember: it's not what you know, it's who you know. There are a million other people who know the same things you know, but how many of them know the same people you do? Thinking of the Olympics, the world's greatest swimmer may live in Tibet, may have never seen a swimming pool in his life, and may not ever discover his ability because he's in the wrong place to use it. Similarly, you can be as smart as you want but if you can't connect your abilities with the people who want to use them, your abilities are worthless.
This is true in business, school, personal relationships -- everything. You wonder why so many terrible movies get made? Because the writer knows producers who trust him well enough to make a movie that will at least break even. Why bother wading through thousands of scripts by unknowns when they can churn out dependable money-makers with the people they've already got? It may sound foolish, but it works.
Likewise, there are a lot of engineers at my job, but the ones who make an impression and get ahead aren't always the most technically competant. Don't get me wrong, you've got to be sharp, but you don't have to be the best technically to get noticed. Remember, every job is a sales job, even if you're just selling yourself. And the way to make sales is to make friends, to talk, to ask questions, to go out of your way to help others, and to let them help you in turn.
It took me a while to learn all this. As an undergrad I was a passive student who didn't much care for interacting with the people in my class or my professors. I finally started wising up when I applied for grad school and started relating lessons I'd learned in the Real World to academia. Then I began really appreciating and considering those relational lessons as work and academics fed into each other. Now, I have a determination to build relationships in every area of my life, because relationships are the keys to success.