Anyone can benefit from companionship which isn't critical, but is a relentlessly affectionate friend. We all need to be listened to without interruption, someone to love us and show us that love and affection, no matter what we do, or how we act. Dogs aren't the only ones who give us this company and love. It is perhaps humerous to think that some of the smallest living things can be joy and happiness to people. Pets can make depressed people feel relieved and help them with through that depression.
Fish are great pets to have.
They won't talk back if you talk to them. They won't curl up in your lap. But they have such a calming, relaxing touch to them. Most people believe this is because of their graceful swimming, and the dull noise that aquarium filters make. Rodents, like rats and gerbils, provide great company.