Gives them a few hours headstart over reverse engineering to get the details of the binary format, and most likely, if the application was password based, which

Gives them a few hours headstart over reverse engineering to get the details of the binary format, and most likely, if the application was password based, which KDF was used. Because even if it was indeed 256 bits encryption, a weak KDF was all too common... In any case, they didn't need the source if their goal was as some say, not to decrypt files a bit faster but to produce a backdoored version.