If I am right, I think you are asking two questions Strength:
Firstly, why is there a position meaning.
Secondly, why is the spread in a particular shape.
Not all spreads have position meanings. I particularly like to use spreads with no meanings and read the cards as a type of story, and without necessarily reading them in order. So in a three card reading I may read the middle card first... or whatever strikes me at the time.
The position meanings are there to give a more pinpointed aspect to the question. So for a "Past, Present, Future" spead the first card would represent the Past etc.
To answer the second part. I don't see the shape as important, only that it may help the reader to remember the positions and may look good for the client.
If anyone else has other views, I'm all ears.
Moonbow*
PS. Its not a silly question. Asking questions is the way we all learn. Sometimes it just takes a while for a response to come along, most of the time it does though.
Keep asking.