Sparring is the most important part of competitive boxing. It is through sparring that the fighter gets a better sense of timing. I like to define timing as the ability to know when to throw a punch so that it lands and know when an opponent is going to throw a punch. While you can develop a good sense of timing through normal partner drills and even pad work, you can perfect it with sparring.
There are two trains of thought when it comes to sparring. One thought is to let the boxers spar at around 60 to 70 percent effort. The other is to let them spar as if they were in a fight, giving all each other has which will improve their stamina and mental toughness. While I do believe there are instances when this is needed, I prefer the first method, especially for those that are new to sparring. The reason being is that when you spar at 60 to 70 percent effort, you can think about trying new things, think about ways to counter, or cut-off your opponent while stalking him in the ring. Sparring allows each boxer to experiment with techniques they learned outside the ring, with their trainer. If you are new to sparring and your partner is trying to take your head off, you will not be able to calm down, will not be able to catch your breath, will not be able to learn anything except how to take a beating. That is not a lesson for anyone to learn.
A good trainer will put a newbie to sparring in the ring with an experienced boxer. The trainer does this, because the experience fighter can see the newbie's punches quite easily. He would be able to defend against them and counter with punches that will not rock the new boxer. This gives the newbie the opportunity to be a bit aggressive and try the things he learned through his time in the gym. Usually the trainer will tell the newbie what to look for, what to try, and sometimes will stop the action to correct a bad technique.
This is as close to an actual boxing match as you're going to get. However, from my experience, sparring is still just another form of training, not actual fighting. In an actual boxing match, you fatigue faster, because of the mental anguish; the excitement, the nervousness, etc.. Also, I do not suggest walking into a gym for the first time and sparring. Even if you are an experienced boxer, you should never walk into a gym from a long layoff and go straight to sparring. You're likely to not only sprain a muscle, but get rocked. Remember, if you want to spar, never do it outside a gym. Always find a professional boxing trainer and a gym that is registered with the local boxing committee for USA Boxing.
* In the sparring videos above, I am wearing the sleeveless gray shirt and red trunks.