Taking a slightly different slant on the question, I'm thinking that 'environment enrichment' is a "two-way street". I've read many articles attesting to the benefits of having pets. So, if there is truth to this posit it would seem that by simply being a pet the animal enriches the environment for the human and the fact that the human provides for the animals needs (perhaps too big an assumption to make in many cases since there is no shortage of stories of animal abuse!) there is also environmental enrichment for the pet.
It is not my intention to skew the discussion, but there are degrees of enrichment and not everyone is in a position to provide for ALL the needs all the time. Providing love and basic care should not be diminished as meaningful contributions to an animal's well-being.
I do not have a pet due to my schedule and travel needs.