I believe that “Why?” is the most important question of worldbuilding. It is, in fact, probably the most important question of any kind of planning. Why.
Everything in your setting should be connected. There’s a reason (if not multiple reasons) behind every action we take, every trend we follow, every dream we have, every political current that emerges. These reasons are often unconscious, but when you build your setting it is important to delve deep into its psyche. You can’t leave it to luck.
“Why” isn’t just about knowing how your setting evolves, however. It’s also a way to research it.
Let’s say your story demands your character is deprived of her belongings upon arriving in a certain society. Why would they do that? Perhaps it’s a society that frowns upon the concept of property. Perhaps they are afraid of strangers and take no chances. Or maybe it’s because she’s a woman.
All of these raise more question, though! Why do they frown upon property? Was there a tyrant in their past who grabbed all the riches? Are they afraid of strangers because they’re isolated? Why are they isolated (do they live on an island? Deep in a network of caves? In floating cities? Do they flee when strangers arrive?) And last... why do women bother them? Is it simple an ultra-patriarchal society? Do they associate women with an evil of sorts? Or do they on the contrary believe they possess a power that allows survival despite having belongings?
I could go on endlessly. It’s what I do when I worldbuild. Knowing the causes of something informs me about it.
So don’t be shy. Use Why. Connect with your four-year-old self again! Your understanding of your WIP can only grow from it.