In what circumstances would you use manual NR in LR rather than Denoise AI/Topaz plug-in?

Or because your computer simply doesn't have the GPU power to drive AI Denoise.

I've noticed many people taking it as an apparent "given" that all visible noise ought to be eliminated from every photo.

But AFAICT lacking all visible noise is a rather 'thin' claim for a photo, and showing some is not necessarily a negative (and may often be a positive). No more an objective measure of merit, IOW, than whether there are brushstrokes visible (or not) in an oil painting. True, avoiding brushstrokes can be important when you are painting a door frame; but we can say that an oil painting is literally "made out of" its brushstrokes. So those genuine traces of its original making that they constitute, can be - and generally are - considered good to see, including in a reproduction.

And grain type noise, sensitively and restrainedly handled, can be thought of in the same light creatively speaking IMO, as those brushstrokes of oil paint. Of course Denoise into a separate file does have its uses, but is definitely not required across the board right now. In due course that functionality might even come inboard and not involve the separate file any more; but let's wait and see on that one.

I probably should have added onto the question "and if so, why is correcting exposure after NR the suggested order, when it makes no difference to the noise". I saw this unanswered question posted somewhere else which what I was also wondering.