Z Chamber 03 Developer Commentary
Developer commentary may include spoilers; I recommend you finish PortalZ before reading further.
This was test chamber two for a while. Since you would get the HEV suit at the end of the first chamber, this gave you reason to try sprinting, and you used stacks of cubes to access the higher levels. In fact, the earliest concept for this level was building bigger and bigger cube staircases until you had enough to reach the exit. Surprising no one, that turned out to be extremely tedious, so the final chamber only has the one cube that needs a cube staircase to retrieve it.
This is one of several chambers with anti-fail measures. The chamber counts how many cubes you have left up here and knocks them down if you put so many up that you wouldn't be able to complete the chamber.