As I pointed out in my previous post, the primary function of the Kaddish, is to indicate and separate the end of one segment of the prayer service, from the beginning of another. But why is this necessary? Here, we must turn to the Kabbalah for an explanation. Have you ever noticed, that just when things seem to be coming together, suddenly something happens that seems to put an end to your great plans? Of course. Everyone experiences that. This is clearly shown in Scripture as well.
 
After decades of apostasy and idolatry in Judah, King Hezekiah brought about sweeping reforms, restoring the Temple, and returning the people to G-d. Yet, no sooner had he completed his great work, after these things and these acts of faithfulness, Sennacherib king of Assyria came and invaded Judah and encamped against the fortified cities, thinking to win them for himself. (2 Chronicles 31:2) The explanation in Kabbalah is that when we have overcome all the spiritual challenges on one level, Evil attacks, and must be defeated, in order to proceed to the next, higher level. This is an essential for growth.
 
In the prayer service, we rise from level to level, from world to world. In all but the very highest world, Evil is ready to block our advance. But we don't give up and run. Instead, we declare boldly, in Aramaic, the twilight language between holiness and evil, "Glorified and sanctified be G-d’s great name throughout the world which He has created according to His will." Evil must flee. ARI z"l interprets a later phrase in the Kaddish "yehei Shmeih Rabba mevorach" (May his great Name be blessed), as "may His Name be made great and blessed". We tell these forces that we are "on to them". G-d is the only power in the Universe. Like darkness, they vanish in the face of Light. They are silenced and destroyed.  

But we must encounter such forces again, and again. And again, we shine upon them the light of reality, and they vanish. We have a Kaddish after the sacrificial section, while approaching the emotional songs and praises. We say it again between the songs, and the recitation of the Shema' and its blessings. When we complete the Shema', and approach the awesome Amidah prayer, we are already beyond their reach. But, as we come down; back to our everyday world, we again encounter the evil forces that seek to stop us returning with our new-found treasures. We make a few stops to again declare G-d's greatness, until we are "home safe". I can think of no greater paradigm for life's journeys...and struggles.