Okay, that was a tad over-dramatic, but it's still true. There are synonyms for most words in our language. In fact, even the word itself has a couple of 'em.
Take a murder mystery, for instance. If a writer were to have the villain "stab" all of his/her victims with a knife in exactly the same way, you'd grow bored of it, especially if they used the same words to describe it. The reader might decide not to finish reading the book, regardless of how much time and effort was put into it. However, the author could use words like "stick", "thrust", "puncture", "pierce", "run through" and numerous other synonyms to convey the same concept, but in more interesting ways. With a little flourish and back story, the readers might not even notice that the villain can't seem to branch out in his/her choice of murder instrument.
However, when using synonyms in your writing, you have to be careful to make sure you're using the right one. For instance, in my example above, I found other words that are considered synonymous with "stab" that just wouldn't work in the same context. There was "incision", which could be used I guess, but doesn't give the same sense of danger or violence. "Ache" was also in there, but that's more what the person would feel after being stabbed. It's probably not even strong enough for the excruciating pain that I imagine would accompany a stab wound. Having never been stabbed before (thankfully), I don't have first-hand knowledge of that physical feeling, so maybe it could be a simple ache. And since most people would probably assume a stabbing hurts more than what the word "ache" describes, I'd steer clear of it, just to be safe.
You do need to be careful with synonyms, though. Like any writing tool, they should be used with care. Using them sparingly and deliberately will help your work move along smoothly, and can make it much stronger and more enjoyable to the reader. If you're unsure whether or not a synonym fits well, take out your trusty dictionary and read the direct definition there. Yes, it takes a bit more time and effort, but as was shown quite brilliantly in this episode of Friends, using too many synonyms can make your writing seem confusing and disjointed.
Synonyms can be your friends . . . if you use them correctly.