Another great question Dottie and not one that is easy to answer. From my experience, herbs are added for both of those reasons - they can provide a food with a good, natural source of nutrients and/or therapeutic properties and, if marketed correctly, they can make a food easier to sell.
As far as feeding philosophy goes, herbs fit in well with a natural, ancestral diet, whichever way you define it. We have all seen our dogs grazing for grasses and while that doesn't really give them much benefit now, it is a behaviour that developed when the variety of wild meadow species was far more diverse and included a huge range of nutritious herbs. They would also have had access to herbs through the stomach contents of their prey and from human cast offs so herbs should arguably be a part of any balanced diet.
Of course, this is not why most manufacturers add them. From my experience, the primary concern of most manufacturers is marketing. If I spend £10,000 on herbs, will I be able to make more than £10,000 back through additional sales? This is sadly the nature of the world as every company knows that a poorly marketed product won't sell no matter how good it is while a bad product can dominate the market if it is marketed well.
For that reason, your average manufacturer will add herbs (or any other ingredients for that matter) not so much for their nutritional properties but because it is a good selling point. If the dog receives a benefit too, then that's just a bonus.
Nevertheless, some herbs can be enormously beneficial for some dogs. Devils claw, for example, has been proven to be at least as effective in reliving arthritis symptoms as many prescription drugs. The problem is that 'herbs' covers such an enormous array of supplements that it would be extremely misleading to say that herbs are a good thing or a bad thing.
There is also the question of herb quality and things like synergies between herbs (some herbs work better or differently in the presence of other herbs) which makes the whole subject far too complex to cover here. If you do want to find out more, you can find a good overview here
Note that I am only talking about most
manufacturers - there are also a number of very good manufacturers that really do strive for nutritional excellence and are very careful about the quantities and qualities of the herbs they add. If in doubt, always ask the manufacturer why they add their specific herb mix.
Just as a final point, beware of foods that just list generic 'herbs', 'herbal mix', 'culinary herbs' and so on as the legal definition "any plants with leaves, seeds, or flowers used for their flavour, aroma, nutritional constituents or medicinal properties"
leaves a lot of room for abuse.