Why do some Muslim women cover most of their faces?

I'm a Muslim myself, I'm young (14 years old) I wear the headscarf and I do cover my arms and body etc. however I don't cover my face/wear a niqab. I Read the Quran but I haven't come across a verse that tells us to cover our faces and all. Was it due to an event in the past or was it just sort of like calm and sense to people? I've always wanted to know the answer to this question, but unfortunately, I don't have any friends who wear the niqab. I'm not trying to be racist or anything of that sort, so please don't feel offended. I'm just very curious. Thank you. And please, if you have nothing nice to say about this- don't say anything.