What is the purpose of "donc" in this sentence? Can the sentence still function without "donc" ? (Par example: "Qu'avait il dû faire?)

[mlamers]

What is the purpose of “donc” in this sentence? Can the sentence still function without “donc” ? (Par example: "Qu’avait il dû faire?)

[Katy_Languages_The-French-Instinct]

It’s like “then”. You could not use it, but we use it a lot