Knowing whether a value is a number or text or date is critical to myriads of application, and CSV specs give no guidance on how to interpret the actual values as numbers or text or dates. You don't have control over both endpoints in most cases, especially when you accept files from clients. Furthermore, they mix presentation and values (is `1.23%` a string or a number? What is the number?)

XLSX separates the value from presentation and gives every value a clear type. If a value is a number, there's a concrete numeric value that is stored separately from the number format. That way there is no guesswork involved, you can figure out exactly what the value is with zero magic.

My take: include a schema when you can and there's a justification and it's practical. IMO even JSON is a (very limited, "name only, no type") type of schema. If it becomes impractical for something you're doing, stop and switch to something less redundant.