Lots of good answers here so I'll not go on about the clean air etc. But here are some other thoughts - if by house plants the questioner means themon indoor foliage plants usefulness (aside from the clean air as sited already and just making us feel better by having them around) is kind of limited. As an aside I do suspect that all indoor foliage plants have the same abilities as the ones used in the 8's studies - don't know that for a fact but I wouldn't be surprised. However if you mean plants that can be grown in the house italic there are vistas of possibilities. You can grow many kinds of fruits and vegetables to eat; many herbs for flavoring tea medicinal purposes and scent; grasses and some herbs are especially good for cats to munch on; you can grow many kinds of plants for gifts or to sell as terrariums holiday decorations etc. etc; you can use plants around and over your windows in place of curtains; plants of a certain shape in a certain style of container can enhance your I could probably go on and on. The point is when you be open to the possibilities plants indoors have many ways of being useful. Once you know how to take care of them few other things give so much return for so little time spent.