Warning: this may sound a little pompous initially, but read on, it gets better
Some folks are more naturalistic than others in their interpretation of what fish "need". From a strictly scientific standpoint, we don't actually "know" what a fish likes, we anthropomorphize (assign human values) . However, based on repeated observations of fish behaviors in the wild and in captivity, a general opinion of what's required for fish to be least stressed has developed.
That's usually what's meant by fish "habitat" recommendations. The, somewhat valid, belief is that the closer we can mimic a fish's natural habitat, the less stress it will show. Having said that, fish may be perfectly content in a very artificial habitat. Most wild fish don't have plastic plants, bubbling scuba-dogs, and resin castles in their native habitats!
A planted tank is just that, a tank containing live plants. You can achieve the same overall effect with plastic plants (I find live plants actually end up costing less!). The primary purpose is to provide cover and a sense of security to the fish. Hardscape structures will provide hiding places and visual barriers, and will provide a sense of security. Purists may argue, but if the water quality is good, the fish seem to be behaving normally, overall stress in the tank is low, then set up and decorate your tank the way you want it! It's your tank.
Realistically, some fish won't really utilize caves and other structures in a pure hardscape as they would live or plastic plants they could weave through. You'll need to observe and tweak the tank scape as you increase the stocking level.