How can power cords make a difference?

I have read many threads about how "wonderful" this or that power cord made my system sound. For the life of me, how in the world can a 3 ft power cord make an improvement? Each manufacturer supplys a power cord.I can't imagine them puting something on thier unit that would not meet thier needs. After all, the power comes from the utility company into your house, you run a dedicated line to your equipment, you put a line conditioner/surge protector between your equipment and now you have clean consistant power then magically you have great music. What can 3 feet of $200 0r more power cord make a differance? Sounds like expensive VODO! Am I missing something?