Totally agree about the others, but this one does make sense and works, I think.
The whole point of the 'ka is a wheel' thing and the final page is that Roland is living a karmic cycle, where he'll live his journey over and over until he's actually worthy of climbing the tower. Every time, the mysterious doors he 'draws' from give him different things/people that make the next journey different. What we saw was his 19th attempt. On this attempt, the doors gave him Susannah, Eddie, and Jake, who all had their own fucked up issues, and then wounded him so he'd be forced to rely on them more. By the end of the journey, he has "learnt a lesson" to trust and engage more with the people around him, erasing one of his personal flaws and becoming slightly more worthy of climbing the tower.
Originally, Roland's childhood friend, owns the horn, and it is dying wish that Roland fetch the horn and not leave it in the battlefield. Roland fails to do this, and regrets it, thinks it's a betrayal of his friendship. In the first book, he reflects on why missing the horn makes him so upset -- the text is "He still had his father's guns, and surely they were more important than horns, or even friends. Weren't they?"
So the horn, to Roland, is a symbol of comradery/friendship/teamwork, and the fact that he left the horn behind is a symbol of his failure to his comrades and inability to keep his oaths to people. The fact that Roland has the horn on his 20th journey means that he didn't make that mistake, he fulfilled his dying friend's wish, because that flaw has been erased from him. It shows that the ka wheel he's trapped in isn't some eternal pointless hell, but that the progress he makes each time is actually kept -- he's making progress as a person and someday he'll be worthy of reaching the top of the tower, or enlightenment, or whatever ka/karmic type thing you take from that.
There is a premonition of this in book 1: the final line has Roland dreaming of the day in some distant future when he will approach the tower carrying the horn. (It also has Roland waiting for events he shouldn't possibly know about, like "the time of the drawing", hinting that he's done all this before.)