Are subtitles (often) necessary?
TL;DR

Subtitles are not necessary.

Large media

Large media and news programs often supply subtitles, because they have more resources. The TV companies in mainland China act like public services, so they have to think about people who are not good at Mandarin, or have trouble with listening.

Cooking programs, talk shows

Some do, some don't. Since these kinds of programs are more free-speaking, they don't have scripts, and it would take some extra work to add subs.

Behavior of native speakers

We like subs, it makes you easy to follow. Especially since we always watch foreign videos with subs, we are used to watching with subs, even when we watch Chinese videos.

Can we understand on the fly?

Yeah, we won't have any trouble with watching videos in Chinese without subs, but that doesn't mean we can recognize 100% of the words, because of accent, background music, rare words, speed, etc. I think that happens among English-speaking people also.