The reasons to cover your phone camera when you aren’t using it overlap with the reasons to cover your built-in laptop webcam since phones are computers. So, I’ll put them into a single, non-exhaustive list:

If you can think of more reasons, shoot me an email[3]!

Link(s):
1: https://www.theguardian.com/world/2014/feb/27/gchq-nsa-webcam-images-internet-yahoo
2: https://ae01.alicdn.com/kf/HTB1sxaIm8jTBKNjSZFNq6ysFXXaQ/Computer-Camera-CoverMetal-Plastic-Webcam-Cover-Slide-for-Mac-Macbook-Pro-iMac-Laptop-Surfcase-Pro-Echo.jpg
3: /about