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I think the purpose is largely aesthetic. But technically what they do is show the plate current, aka the bias, on the power tubes. That is useful in two ways: one is generally the needles will be perfectly still, but as you start to approach clipping they will start to dance, in time with the music.

Also, while the absolute number on the dial isn't very important, if you see it go down it probably means your tubes are getting old and you should consider replacing them. But my (some what limited) experience is sound quality starts to diminish before the meter reading changes. Steve suggests every 6 months or so to swap out your current tubes with new ones and compare the sound. If the old ones still sound good than continue to play them. If the new one are better than thank the old ones for their service and leave the new ones in the amp. That makes a bunch of sense to me.

There are really no user adjustments possible, besides switching tubes, but if you see the meters go to zero, or get maxed out that means you've got a problem.