Hi lainmac,
Thank you for an opportunity to write about one of my favourite numismatic subjects! A similar discussion exists from a few months ago here:
https://en.numista.com/forum/topic42696.html
Essentially, summing the information posted there up in my own words:
After WWI there was a shortage of circulating coins in France, due in part to the economic situation created by the war and the hoarding of silver coins by people after the devaluation (and end) of the Franc Germinal, the LMU standard Franc that had existed since 1795. This shortage is also the primary reason for so many municipalities minting their own tokens (see French Notgeld) for circulation in the 1920s.
Thus from 1920 the Chambres de Commerce stepped in for the Banque de France, and had (what were technically) tokens of 0.50, 1 and 2 francs minted for circulation, bearing the words "Bon Pour" (Good for) to mark their lack of intrinsic value (owing to the devaluation mentioned earlier). Countries with currencies pegged to the Franc (Monaco), French colonies (Tunisia) or countries in similar economic situations (Belgium, Luxembourg) also issued similar coinage bearing "Bon Pour" around the same time. By the end of the 1920s, with the re-stabilisation of the currency (known as the "Franc Poincare") "official" coinage issues (without Bon Pour, and not from the CdC) resumed.
Essentially, the difference between ordinary French issues and the CdC coins is the fact that they were not issued by the Banque de France (because of devaluation), unlike the regular issues that preceded and succeeded them.