This is to check whether or not the item can be thrown by a player. For example, Material.ENDER_PEARL.isThrowable() would return true.

I was going to use this to get the ItemStack an enderpearl was thrown from by a player. This is because the itemstack will have persistent data on it that makes it a special enderpearl. In order to do this though i need to figure out which one was thrown: mainhand or offhand. If the pearl is in offhand but the mainhand has something that is throwable, that mainhand item is thrown.