Audio and subtitles are complementary.

If you’re having a conversation, or doing some other task that makes sound, or scrolling social media and a video starts playing, there could be a noise that momentarily covers up the audio and you miss something. If there are subs then you can quickly glance to see what was going on.

Listening to spoken dialog allows you to look away, but subs let you catch back up if you miss something. They cover for each other.