[Grammar] Will, going to, will be ....ing

[beachboy]

1) On Saturday, I'll visit my sister.
2) On Saturday, I'm going to visit my sister
3) On Saturday, I'll be visiting my sister.

I understand that, in affirmative sentences, "will" refers to a decision made at the time of speaking, "going to" refers to a intention to do something, and future progressive refers to arrangements. But I don't understand why a native speaker would choose to ask "What will you be doing next Saturday", instead of "What will you do" ou "What are you going to do next Saturday?". Thanks.

 

[beachboy]

Yes, the article is rather interesting and helpful!! Actually, I meant to simplify things when I put down those sentences. But what I really can't understand is when to ask a question using either "will", "going to" or "will be ....ing", as the person who asks the question doesn't know much about the other one.