They did play a significant role in defensive warfare on many occasions (we have depictions of walls being attacked and defended in much the manner you'd expect of, say, medieval sieges), but I think it should be kept in mind that walls are often meant for internal control as much as external defense. From very early on it's clear that monumental architecture, especially grand walls and gates, served as a testament to the might and power of the king (see, Gilgamesh's bragging about the walls he constructed around Uruk, inscriptions left in walls praising the various kings' grandeur and piety, etc), to impress upon his subjects the essential stability and power projected by Kingship and the dynasty. The visual impact of towering, massive structures raised against the flat and largely-featureless landscape of Southern Iraq probably can't be overestimated today. It's an extremely potent proof of the validity of the monarchy, both for its practical function and the grand organization of labor and resources that its construction represents.

Just to throw some hard figures in here, the walls of Uruk were 40-50 feet high at the turn of the 3rd millennium (Jemdet-Nasr period)