Usually your friends and family, as another poster above said, can see things that either you are a)choosing to overlook or b) have not noticed about your significant other. I've yet to find a GOOD friend or relative that would lie to me about a person I was dating. Often times, even their "hunches" would be good ones...meaning, if they got a bad feeling about someone, it (unfortunately) turns out to be true.
I like to listen to how friends and family feel about a SO, because they know me well, better than my SO. It may or may not affect what decisions I make in the relationship, however, but it's important to know if potentially, somewhere down the line, your SO may have some problems/issues with a CLOSE relative or friend...