I'm just throwing this up as a basic reason why I think finding themes (or overarching questions to your books) and symbols (things that relate either directly or indirectly to the theme) early on is helpful toward long-term success.
Note: I am not an author. I'm just a guy who sees a lot of 'how do I do X' on this forum, or 'why doesn't my character sound 'real'', and who found themes and symbols as great (and underutilized) ways of fleshing out these characters. Take my two cents with a pinch of salt.
Anyway, the reasoning behind this is simple: if you give a question, your characters have to give an answer. I cite My Hero Academia's 'What does it mean to be a hero', to which most characters (Deku, Bakugo, Stain, etc.) have given their answer. This forces you to get a better grasp of their character, and their place within the overarching narrative of the story.
Contradictory answers facilitate conflict. For instance, in my book, the question is 'what defines 'yourself'?', to which one character answers 'I am whoever I want to be', while an institution defines it as 'you are whatever makes you unique'. Thus, the character and the institution are not only at odds, but I know exactly how and why they are at odds, and I can use this conflict as a baseline to build off of.
Because, unlike most elements in books, theme is seemingly immutable. Using it as the bedrock of your story, so long as you have a good theme, has at least led me to writing some of my most concrete characters, and, as a direct result, chapters, in years.
Symbols work in much the same way. My biggest symbol right now is clothing, which symbolizes self-expression. Now, going back to the above example, we can say that the character is a weaver, who makes her own clothes (thus choosing what she wears), and the institution (who doesn't allow self-expression), would use uniforms. We have now brought those conflicting themes out of the esoteric mumbo jumbo, and into the pages of the book.
They can also work in minor instances, where you need dead time. One of my scenes needed a bit of space in between 'characters walk out of concert' and 'characters realize their car has been stolen', so I had the one character buy the other a t-shirt, and the pair are talking about it as they walk to where they think their car is.
The drawbacks, as I've come to view them, is that a.) everything takes longer (some actions have really driven me down the rabbit hole in terms of depth, for example, cosplay) and b.) this only works if you genuinely have a question you want answered, and can be answered differently by different people.
Anyway, you could think I'm completely off the mark, that this is horrid advice to give to anyone. There are three rules to writing any great book, and everyone disagrees on what they are. So what's your opinion?
TL;DR: IMO, symbols and themes (more specifically, your book seeks to answer a complex question), can lead to more well defined characters, more complex plots, way better scene composition, but only if you put in the time and effort into exploring your question, and the ramifications therein.
Source: reddit post