Plants improve the air quality (apparently) and their presence is known to have a calming effect, as green is a relaxing colour for humans. It's also nice to have something living in the artificial environment of a flat/house, particularly in heavily urbanised areas.
Cushions are useful for supporting your back or head when you're sitting or laying down and the different textures and colours provide pleasing stimulus.
Ornaments could be for sentimental reasons, tactile responses, the way they catch and change the light in the case of glass or crystal or because they accentuate or contrast to other colours, textures and shapes in the room.
Flowers, like green plants, provide colour, scent, texture, form and promote positive associations.
Many of us have lived in completely bare rental properties where everything was magnolia/white or beige, bare bulbs casting harsh light and a sense that the place was not a home, it was somebody else's property and we were only there temporarily, particularly if financial issues made it impossible to keep warm or purchase luxury items to increase comfort. Once you get into a position where you can actually decorate however you like and get the soft blanket that feels like a hug, a lot of people (most in my opinion) go straight ahead.
Humans have a fundamental desire to decorate and embellish things - there wasn't any need to paint wild bison on cave walls, but early peoples found out that certain substances made marks - so they used them. We didn't need to discover gold, silver, make bronze or decorate ourselves with feathers, stones or blossoms, but we did. I don't suppose a Late Neolithic - Early Bronze Age woman stuck a badger's skull full of daisies on top of the hearth when it was out, but she probably liked having her hair braided, some seeds dried and pierced to carry a strip of sinew through and worn around her wrist and adored the decorated corded ware pots, never mind the decorated beakers that came with the Beaker People.
We like 'nice' things. Because we're human.
(although plenty of animals also collect things that interest them - hence the Magpie connotations - or the amalgamation of feathers, broken shoelaces, ping pong balls and toy mice that are currently underneath my settee, as the TwatCat doesn't often play with them, but likes collecting things that make him happy I'm just glad there aren't his favourite chew toys under there, being real baby mice).