Here's why:
* Old shoes: Over time, shoes lose their shape, become cracked, and their soles wear thin. They're no longer comfortable or functional.
* Worn out: This phrase indicates that something has been used extensively and has reached the end of its useful life.
The simile suggests that the thing being compared is in a similar state to an old shoe - it has lost its value, appeal, or effectiveness. It might be:
* A person who is tired and exhausted: "After working all night, he felt as worn out as an old shoe."
* A relationship that has lost its spark: "Their marriage was as worn out as an old shoe, with no passion or excitement left."
* An idea that has been overused: "That argument is as worn out as an old shoe, I've heard it a thousand times."
In essence, the simile paints a picture of something that is spent, discarded, and no longer holds any significance.