The new Outlook + Planner/To Do combo is still a mess. Outlook’s slower and clunky, Planner and To Do don’t sync well. Bugs and sync issues are common. If you want real productivity, switch to tools like Todoist or Trello.
The short answer is that "Tasks" is To Do's inbox and the other two lists are not necessary for GTD at all. There are also other smart lists that I have disabled.
More details:
"Tasks" is where an item goes in To Do when it has not been assigned to any list. So I use it as an inbox which I clarify same as my in tray or emails.
"My Day" is a tag that can be added to any item, whether it is on a list or just in Tasks. If you tag it with My Day (or just create the item in My Day), then it will appear in this valuable real estate at the top of the app. I don't routinely use this to plan my days, however sometimes I might enter items here if I have a logistically tricky day and I need to plan out a sequence of dependent steps involving time pressures. Or I might tag a few items from my action lists here as options for the day. More often I just leave it unused. Occasionally I use it as a notepad to type something like a draft sms. This can be handy because I can type on my laptop and then use the text on my phone. At the end of the day, items on My Day lose the tag and remain in Tasks or whatever list they were on already.
"Important" - I use this sometimes to highlight certain projects that I want to cast my eye over in between weekly reviews. I just mark the star next to the item on my projects list and it tags it as "Important" and shows it in the Important view. I usually would only have 4 or 5 items on there. I am a teacher, and at the time of these screenshots I was using it to highlight projects that I wanted to make progress on during the school holidays.