I believe it's a symbolic reflection of their knowledge and wisdom. Low to the ground, you can't see much. There are obstructions in the way. What lies beyond

I believe it's a symbolic reflection of their knowledge and wisdom. Low to the ground, you can't see much. There are obstructions in the way. What lies beyond those mountains? Ascend high and find out. Atop the tower, there is nothing obstructing your vision. You can see for miles. One who has scaled the tower is all-seeing. Wizards have reached great heights of knowledge and wisdom. They are all-knowing, or all-seeing, as is the one who's scaled the tower. Now why a tower rather than something like a whole castle? Towers are a bit more humble, independent, and potentially isolated, as the wizard himself often is.