The 1918 Illinois Statehood Centennial Half Dollar is one of the coins of the classic-era series that, on a reasonably often basis, appears in the marketplace in circulated condition. Why? "Blame" it on US President Franklin Delano Roosevelt and the Bank Holiday of 1933.
As the Depression worsened and thousands of US banks failed/went out of business, US President Franklin Delano Roosevelt (FDR) declared on March 6, 1933 (via Proclamation 2039) that all US banking transactions, including deposits and withdrawals, were suspended for a period of one week - access to funds was eliminated. (Note: FDR was inaugurated as the US President on March 4, 1933 - just two days before.)
The "Holiday" period was used by the Federal Reserve and the US Congress to develop a plan that would allow closed banks to re-open and for banks to be be able to issue money based on their "sound assets" vs. gold reserves. Not all banks would be allowed to re-open, however, only those that were deemed to be at very little risk of failing or those that presented a failure risk that could be managed with a reorganization of the bank's business. Approximately 4,000 banks never reopened due to their significant insolvency; those with deposits at these banks lost their money (estimated at ~$7 billion).
The Emergency Banking Act was passed (i.e., signed into law by FDR) on March 9, 1933. The Act sought to stabilize the US banking system, expand the US President's authority during banking/financial crises, set the terms for banks to reopen and established mechanisms for banks to gain access to additional currency.
During the "Holiday" most banks took inventory of their assets, including the physical currency held in their vaults - this included paper money as well as coins. Within multiple Illinois banks (many in Springfield) were quantities of unsold 1918 Illinois Statehood Centennial Half Dollars - approximately 30,000 coins. It is believed that the majority were sold to coin dealers at a small premium over face value, but a significant number were also placed into circulation. It is primarily these coins that have helped give rise to the availability of circulated examples of the 1918 Illinois half dollar.
1918 Illinois Statehood Centennial Half Dollar

For other of my posts about commemorative coins and medals, including more stories about the Illinois half dollar, see:
Commems Collection.
I don't have a circulated 1918 Illinois Statehood Centennial Half Dollar in my collection to illustrate here, but if nickelsearcher stops by, he might offer up images of his!