Why We Need PCTFREE instead of PCTUSED at Index ?

Basically, a table is a "heap".  We stick data WHERE EVER. an index is a structure, we have to stick data where it belongs.

When we take an index block off of the free list -- it is "empty".  We will then totally fill it up  until we have to split it.  It only goes back onto the free list when it is empty again.  We cannot  put a block on the freelist unless it is empty -- the fact that the INDEX IS A STRUCTURE, prevents us from sticking just any data on the block.  Only data that can go onto that block can go there.  
It is just "not a heap".