It helps you plan because not everything can be done now. The “when” makes you think about a time for getting it done. You have things you can do now and you plan for all the other things you need to do. You are not overwhelmed by a long list of things to just do now.
Its other strength is that it’s harder to ignore a question than a command. Humans feel compelled to answer a question.
It’s a complex question. It makes you think.
You need to know, “Can I do it now?”
You will need to justify, to yourself, a “no”, so,
you will have also asked yourself “Why not now?”
And every-time you think of “not now,” you remember you will need to decide “when.”
You may decide, it’s simpler to just do it now.