I'm not expert of this time period.
The French was soundly defeated at Crecy and Poitiers, the king was prisonner of the English.
But Edward III attempt to seize recent was a severe failure, so he was not precisely in a very strong position.
I think the main reason was to gain time. Charles V needed time ro secure his power (he was regent), he had to face the revolt of Etienne Marcel and the wealthy Parisian merchant, and Charles le Mauvais and the Jacquerie.
With the treaty, and gained 9 years of peace, to reorganzie the country and restore stability. He send Du Guesclin in Spain with the free company, to fight the English allies there, and gain a new ally for later.
The treaty of Bretigny tied the hands of the English, who could not start the war again without breaking it, and thus losing the many concessions they gained from France. The failure of Edward III last expedition showed that pushing their luck to far may have been unwise.
The treaty said that the authority of the English king on new territories would be effective only after the transfert of all the territories, and the English would then renounce his claim on the French crown.
But it was a trick from Charles V. He deliberately slowed the paiement of the ransom, and the transfert of the territories, so the English authority was not effective (and saying in the same time "but, I'm not doing it on purpose, look you still can't renounce your claim, it's upset me as well).
Why the time bought by this tactic, he reformed the tax and monetary system in the country (creation of the Franc), Du Guesclin defeated Charles le Mauvais (Cocherel), and when he went in Spain with the Free companies, the latest stopped to pillage France.
So in effect, Charles V maange to restore stability, and gain again the favour of the people. As Edward III made English the official language of the court (before that it was French), the anglophoby increased in France.
The English were stuck in Spain to help Peter, Charles secure a peace with the Holy Empire, and with Flanders.
When Charles V was strong enough, he started the war again inn 1368, but this time he did it in a way to have the law on his side. During 10 years, the army reformed by Du Guesclin will adapt new tactics and slowly regain territories, while the "Chevauchées" of the English made them less and less popular.
Overhall, the treaty of Brétigny can be seen as a wise move from the French regent, who promised a lot to the English to buy enough time to reform, reorganize and win later, while actually giving little.
To sum it up, the French's regent was smarter than the English kinds and was thinking on a long term strategy.
That's one of the strength of the French: the hability to be seriously defeated, by time, reorganize, reform, and come back to sit at the table of the winners at the end.
Look at the 100 years wars, the revolutionnary wars, WWI or WWII. Ok, it doesn't always work...
