Actually, I always found this a bit odd. The first thing said about the lamp-post in LWW is that it was about ten minutes' walk from the wardrobe, and ten minutes' walk in a forest is a very long way. And later when all four get in, they wander off away from the lamp-post such that Edmund says, "Oughtn't we to be bearing a bit more to the left, that is, if we are aiming for the lamppost?" which suggests that the lamppost was fairly easy to miss, at least in daytime. It's really not a terribly useful signal for anything, unless there was a very clear path from the lamppost to the wardrobe - in which case one wonders why more people didn't end up going through the wardrobe! But then the Witch seems to know that the lamppost is a pointer to "the world of men" and directs Edmund that way. So I feel Lewis's description of the lamppost is a little confused.The lamp post also acts as guide post or a landmark so that a visitor can find their way in & out of Narnia.
It does, but TMN was written after LWW, so this is Lewis giving some backstory that he probably hadn't thought about at the time he wrote LWW.I just read somewhere that it represents a "connection" between Narnia & the real world (Queen Jadis twisted the arm off of the lamp post in London, tried to hurt Aslan with it in Narnia, it fell on the ground & grew into a new lamp post during the creation).