When JLG introduces Ariadne to the penrose staircase, he says that you must utilize illusions to create the boundaries of the dream....so the dreamer doesn't know they are fake. That is another key skill, utilizing those illusions to create the artificial borders of the dream, basically tricking the sub-conscious. Further, the architecture of the dream must not only be convincing, but complicated enough that it gives them ample time to hide/navigate away from from sub-conscious threats if need be.

Further, in Leo and Ariadne's first convo(the one at the Parisian cafe), he explains that dreaming is a state in which you simultaneously create and perceive.....so it is the architect's job to get right in the middle of that. If you remember, he draws the diagram with the circular arrows pointing at each other and draws a line through the middle. So it is the architect's job to create the basic structure in which the dreamer will perceive his or her subconscious thoughts.....otherwise, who knows in what kind of world they would end up. Also, the architect is required because they must install certain features such as a safe or safe-house where the subconscious will naturally store secrets due to the perception of safety provided by those kinds of constructs.