This may be a stupid question but I notice there is a knurled bolt at the back of each chainstay. None of my other bikes have this.
I assume it is to move the rear wheel either backwards or forwards but I don't know why or where I should adjust it to. The bolt doesn't seem to loosen to let the wheel go
all the way to the back.

Those bolts should be most of the way in, in most cases. If they are sticking way out in the back, they can easily get bent, and the wheel may be too far back in the drop outs. They are there so you can adjust them to center the rear wheel. When the rear wheel is pulled into the dropouts, you want it to be perfectly centered. They make setting the wheel quick and easy. When you loosen the quick release and pull the wheel back so there's even pressure on both those bolts, check for how the rim is centered between the chain stays (by the seat tube). Adjust as necessary.

You are correct about the usage. You can adjust it to wherever feels best to you as long as the quick release has enough drop out to bite into.

With your last sentence, are you saying that the bolts don't move at all or are you saying that they move but not all the way to the back? If you want the wheel to go all the way back in the drop out, you need to pull them all the way out. I usually do that because I like long chainstays.

Does this not tighten or loosen the chain also?

But wouldn't the wheel be centered if those bolts weren't there at all?

It would have a very small effect on the chain if you are using a rear derailleur. If you are running it single speed, you could use them to set the chain tension.

It would probably be close, but the drop outs can get bent a little one way or the other. Also, if you want to make your wheelbase shorter, those bolts help you to get it exactly the same each time you put the wheel back on. As I mentioned above, I remove them. If I need to adjust the wheel to center it, I just do it by hand. Since I am not a racer changing flats mid race, I don't mind taking an extra minute.

Yes on both counts. On the last point, many bikes are ever so slightly out of alignment from the factory. A tiny discrepancy between the dropouts is magnified by the radius of the wheel. So even without a dropout getting tweaked, they're useful most of the time. I like to leave them in because it does make it easy to quickly center the wheel. Plus it just looks right to me, but then I also usually use a cap on the valve stem of my tubes.

EDIT: You also mentioned that the bolts don't seem to go all the way out. Most of the time, the are put in from the "front" of the dropout (the side towards the fork). Then they have a cap that screws onto the exposed end. Unscrew that cap, take the wheel out, and then unscrew the bolt so it moves towards the fork, and it should come out.

Thanks for the explanations ,

It should be, but they also serve to position the cogs properly above the derailleur for optimum shifting. That position usually isn't with the axle all the way back in the slots. In a race, you could quickly put on a spare wheel and have it be perfectly aligned. Modern bikes have vertical lugs, so it isn't an issue.

The position of the rear axle to the rear derailleur pivot is crucial to precise shifting. It is all about shifting geometry. A millimeter or two in alignment makes for that elusive 'buttery-smooth' shifting (we're talking friction shifting here!)

I learned something new. I'll pay more attention on my friction shifting bikes. In looking this up, I found this old BF thread where none other than Sheldon Brown weighed in.