How important is it to use data in the Mains answer writing?

In my opinion, yes it is really important to use data while writing answers in UPSC Mains Exam as facts are there to represent the absoluteness of knowledge, its certify the very base and also there to support your perspective, when you draw a hypothesis first you need some random facts in a raw form, then you arrange them in a manner that they looks like, they are supporting your answer, it's kind of acknowledgement to make your answer sound, meaningful and concrete. Basically facts change over a longer period of time, but figures and data are time bound, they need to be revised with respect to time. Facts and figures also help you in drawing conclusions, building models and comparing them, these act like some kind of proof and evidence. So I will suggest you to follow https://iasscore.in/data-story as it will definitely help you to score good marks in UPSC Mains Exam.

Good Luck!