A table can only have one autonumber field, so it's often used for the primary key.  To use autonumber in other ways, be sure to watch Richard's video titled "Autonumbers Are Not For You".  You can guess from the title that Richard believes you shouldn't use autonumbers for anything other than primary key fields because of the inherent restrictions of autonumbers.

In my old job of 20+ years, I never found any other purposes for autonumbers because in real life we use sequential numbers that are highly structured (with dashes, octets, alpha-numeric characters, etc.), a feature that autonumbers can't provide.

We rarely see any real-world numbering scheme that is simply 1, 2, 3, etc.  Phone numbers are in 000-000-0000 format.  Social security #s are in 000-00-0000.  IP addresses are in 000.000.000.000.  Driver's license numbers usually have 1 alphabet + several numerals, etc.  These formats are designed to look (mostly) unique to one another to benefit the human eyes.  If they all looked like 1, 2, 3, etc, it would be confusing.  That's why a numbering scheme like 1, 2, 3, etc. is almost never used in real life, hence the lack of usefulness of autonumbers.

What Kevin said. :)