Speed. You can look through disassembled code and find the algorithm in due time, but having the source code could make your job take literally five minutes if all you need is the right parameters (e.g. bcrypt with cost factor 11 as input for DES). Knowing those, and assuming it's reasonably small, you can just fire up your cracking station at that point. If it's too strong, you'll have to look for weaknesses in the code, which is still much faster to do with source code than with assembly.

For backdooring, getting it to compile might take longer than putting the right jumps in and linking the desired dll, depending on how standard the compilation toolchain is and how experienced you are in backdooring.

This is all assuming that they have the program in the first place and not only the encrypted file, though it sounds like they did.