It's hard to explain for me so I'll write an example.
When you make a block and you want it drop always air, you set the custom drop as air.
However, if a player breaks the block with the Silk Touch enchantment, the block drops itself.
And sometimes you really want that block to never drop itself.
So, it would be cool to add a "Is affected by Silk Touch?" option to blocks drop.
This is also useful for preventing cheat: for making a block that drops more than an item, you set a custom drop and a 'when block broken by player/explosion" event/procedure. However, if the player has a silk touch tool, he will get the extra drops+the block itself and he will be able to get unlimited resources.
If you add this feature a lot of bugs will be fixed