Originally Posted by
Brainslinger
However, even if the horn was a reward, I don't take that to mean his progress in the previous loop was any less important. Ultimately I see the horn as being the positive side of being a Gunslinger. The horn provides a rallying call to ones men and therefore can be seen as a symbol of ka-tet, indicative of Roland's need for others to succeed, something he really learned in this loop.
At the start of the Revised Gunslinger he ponders on having left the horn behind at Jericho hill, and then speculates "Surely it's the guns that are important?"
True the guns were important. They saved his life and his ka-tet a number of times (whilst ending others) but in the end they are just machines of death, the negative part of being a gunslinger. That's why just leaving the gun at the foot of the Tower was not enough. He needed the symbol of the positive side, manifested by sounding his horn in the field of roses as well.