To me, if players are extremely knowledgeable about every aspect of the world in which they are playing, you lose some of the ability to use mystery and surprise as part of the game experience. I don't like running games that are liked souped-up versions of video games and MMOs, where casting a spell this way always gives the same exact result, an armored soldier with a battleaxe will always defeat a less-experienced guy with a knife, every member of X race is violent, and every monster fights the same way as every other member of his/her species. I like to take all the different variables and situations into account.
If the characters have never been to a particular distant land, have never met anyone who has visited it, and have never read about it, they shouldn't know what to expect. Even if they have talked to people and/or read about it, there is no reason to suspect that what they have learned is complete or accurate information. There is no reason to suppose that their own individual experiences in that land (assuming they reach it) are indicative of the experiences that others have when they travel there, either.
(Just to use one real-world example, having done a lot of international travel, I have encountered many occasions where my companions and I have returned with vastly different experiences and opinions of the places we visited.)
That's one reason that I tend to prefer to use original species (I don't like the term "monsters" in this context) in my game worlds. Most experienced players have a certain set of assumptions about goblins, orcs, dragons, gnolls, etc. I like to hit them with things that are complete unknowns. That way, when they encounter something in the woods (or wherever), they don't know if it will be friendly, indifferent, or hostile. If it's hostile, they don't know what weapons or substances will work best on it. They don't know if it is likely to fight to the death, or flee if things get rough. They don't know how intelligent it is, how tough it is, or how best to communicate with it. They have to learn these things through trial and error, and live with the results and consequences, whatever they may be.
When I do use a standard "monster," it may or may not have the same stats listed in the official books. It all depends on whether I agree with those stats or not, and what roles that species tends to play in my world.
World-building is half the fun for me when I GM, though. I know that there are people who aren't as into that as I am, and prefer to rely on already-established worlds. There's absolutely nothing wrong with either approach. I look for players who can enjoy my style of GMing, and (when I play) for GMs who have a style I can enjoy, whether it's similar to my GMing style or not.
