The yidams also appear in so many different ways in order to symbolize that the whole spectrum of our clinging to impure appearances is purified. Now, we have a dualistic perception and are always thinking in dualistic categories. Therefore, we are not able to relate to the ultimate yidam and we need something which represents him. The many forms of the yidams which we know from pictures are in that form symbols for the ultimate yidam. The meaning of it is as follows.
All appearances arise in a mutual dependence.
Something arises at a certain time, stays for a while and disappears again. The two phases of meditation are used in order to symbolize that the principle of arising and disappearing is carried on to a pure level. The arising of a deity symbolizes that the clinging to the arising of the commonly experienced world is purified. In the developing phase one deals with it in a way that one does not think about whether one exists or not, but one simply disregards this question and visualizes oneself in the form of the deity. The visualization of the yidam in space in front of oneself works in a similar way.