why does ffmpeg rewrite images when converting to video code example

Example 1: png sequence to mp4 ffmpeg

ffmpeg -r 60 -f image2 -s 1920x1080 -i pic%04d.png -vcodec libx264 -crf 25  -pix_fmt yuv420p test.mp4
-r is fps
-crf is quality, lower means better
-s is resolution
-pix_fmt is pixel format

Example 2: linux create a video from images

ffmpeg -framerate 5 -i img_%04d.png out.mp4