Particularly if you are on the pro audio side. Consumer audio can ignore pro-audio for the most part. However everyone on pro-audio needs to do something consumer audio once in a while, if only to run a web browser.
Exactly. If you're working on a project in Ardour and want to quickly watch a video on YouTube, stopping Jack and starting PulseAudio was a bit of a pain the last time I did that.
Yes, you can configure PulseAudio as a Jack client, but the session handling is also a bit messy. (I used to have a PA -> Jack setup on my work computer just so I could use the Calf equalizer / compressor plugins for listening to music. I dropped it again after a while, because session handling and restoring wasn't always working properly. But that was around 6-7 years ago, maybe it would work better nowadays.)
Setups for professional music production. This usually requires JACK (https://jackaudio.org/) as audio server, which allows synchronizing and connecting multiple applications.
For example, you can have Ardour (https://ardour.org/) as DAW, but use another application like Hydrogen (http://hydrogen-music.org/) for creating drum samples. JACK connects the two applications using a virtual patchbay that allows using Hydrogen as an Input for Ardour. Essentially any application can be an input and/or an output.
JACK also provides synchronization using a "master clock", so that Hydrogen starts playing as soon as you hit the "record" button in Ardour.
Many people also use a Linux kernel optimized for low latency audio.
With PulseAudio, these things are not possible. On the other hand, consumer applications like web browsers don't usually offer direct JACK support. So bridging is necessary, by using PulseAudio as a JACK input.
