The type system depends on the kind of data you want to keep around in your columns, which could be a mix of types, and it could be all strings and NaNs, for example. This is common with DataFrames. There's a huge number of possible combinations of data you could be reading in that you need to clean up, reshape and then analyze. And you might be making decisions on how to do that in the middle of your wrangling. It's just like what you can put in Excel or a CSV.