With some exception like one jobs that is more important is resource starved by for example network while the low prio job isn't, then you can have the network starved job be 'ready' to receive resources right away while the low prio takes up any left over CPU cycles. And base priority is not really the place to determine what jobs get more resources if two are running anyway. I would argue that an application that intends to use 100% CPU like ffmpeg don't benefit at all by being set to more than low, yes maybe 1-10% if the resource it could fight back for at the expense of everything else. The only reason to set anything above normal in my opinion is video player and OBS and those type of real time applications that would stutter if they where not getting resources when needed right away instead of waiting. But then manually submitted jobs has to follow the same setting.

I suppose that if you never use anything above normal in regular use then it's fine, use caution with higher. You are correct, I was manually submitting the files, which would explain the high priority then.

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