According to DW, which is arguably an European (German) source, but probably not very French enamoured:
Guinea quit the CFA monetary zone in 1960, followed by Mali in 1962 and Madagascar and Mauritania in 1973. But their decision to leave put their currencies into such a tailspin that no one has been seriously tempted to follow their example since.
The CFA franc is pegged to the Euro (previously to the French franc) and guaranteed by the French treasury. Defenders praise the credibility and stability of the currency and point out that no CFA member has experienced a major financial crisis.
One indication of the CFA zone's attraction is that Mali chose to rejoin it in 1984, while Equatorial Guinea, a former Spanish colony, and Guinea-Bissau, a former Portuguese colony, also later became members.
The article also goes into some criticism of CFA, but you asked what were the pros/incentives. In a word, it seems "stability" is the main attraction.
Likewise, a BBC article, mostly critical, also noted
But there are economic benefits of a stable and easily convertible currency, says John Ashbourne, senior emerging markets analyst at Capital Economics.
"Inflation, for instance, has tended to be much milder in the CFA countries than elsewhere in Africa."
But that seems to be it; grow-wise it doesn't seem to have made much difference:
Average GDP growth - the rise in the total value of goods and services produced - of CFA countries and the rest of African economies is, indeed, fairly comparable over the past few decades but that could be due to a range of factors not just the currency.
(The BBC article has an IMF graph for the latter non-difference.)