To comprehend expired domains, and their value, an understanding of the discrepancy between a domain and a website is needed.
A useful analogy is to liken a domain to a business telephone number, and a website to a business office.
People can use a telephone to enter a business telephone number to call up the company and place orders, in the same way
that they can use an internet browser to join a domain to look at the business website and place orders.
With the telephone, the business pays the telephone firm. If they don't pay then their telephone contacts are cut off, and after a while, their phone number gets reassigned.
This is equal to domain names. A company will pay to register a domain for a typical period of time - normally 1 or 2 years. The company can then pay again to continue the registration for another set term.
If the company quits paying, then the domain expires and becomes open to be registered by anyone else.
Suppose you owned a business where telephone orders were an essential part of your firm.
Now imagine one of your local contenders goes out of the industry- your orders would leap if you could redirect the telephone callers from the competitor's telephone number to your own number.