Q: Why ?

A: Why not !

- using Firefox under Jack you must recompile the source code
- using Discord under Jack is not possible !
- using Jack client under OBS, you experiencing Xruns !

The best option (for me)  is to bridge Pulseaudio and jack

My Complete Audio Setup :

- Extra Package for realtime privileges or edit /etc/security/limits.conf manually
- Classic Mainline Kernel (not the RT !!!)  most RT capability are in mainline kernel for long time yet
- Ardour as the Main Mixer and routing solution
- Qjackctl for Setup Jack & Pulseaudio Bridge

QJACKCTL SETUP :

- REALTIME             (box checked)  - Standard kernel have RT Capability ! 
- Select INTERFACE Hardware
- Choose SAMPLERATE       (48000 depend HW)
         Frames/Period    (256   depend HW)
         Period/Buffer    (2     depend HW)
- Support Midi in JACK (SEQ)  if needed  (Alsa have Midi Support)
-
- Interface D-BUS      (box checked)
- Interface JACK D-BUS (box checked)
-
- Execute Script on startup     : "pacmd suspend true"   (to disable pulseaudio)
- Execute Script after startup  : Custom Script to start Pulseaudio and create Named bridge
- Execute Script after Shutdown : "pacmd suspend false"  (to restart pulseaudio after jack if poweroff)