I copied all the videos from my iPhone to Photos app of Mac, and deleted the videos from iPhone. But sometime later, I wanted some videos on my iPhone to show it to my friends.
Tried to use iTunes, but could not find a way to copy to iPhone, same with Photos app. But finally figured out that if you send the video files via Airdrop from Mac to iPhone, it will be saved in Photos.
Comment if there is any better way.