[Buggy]

Ok, so your girlfriend has kept 1 betta in 1 bowl for 1 year at 76F and it has done fine. Great! But not everyone keeps their house at a constant comfy tropical temp therefor you need a heater to keep the TANK WATER at that constant optimal temp. What do you think would happen to that fish if the room temp suddenly dropped to say 50F?
The information these people are giving doesn't come from just one or two personal experiences. It comes from reading and researching hundreds of resources written by hundreds of professional fish keepers and breeders over several decades. The scientists and marine biologists that observe and record this information aren't supporting the sales of aquarium heaters. They are bringing to the public awareness the natural conditions in which these creatures live and thrive so that those conditions can be duplicated in a controlled environment.
The same thing goes for that goldfish. It can withstand temp fluctuations of several degrees but it would take only one time of the temp jumping up to 90F to cause serious problems or even death. The heater is not just to MAKE the water the proper temp but to KEEP it there. Even animals kept in zoos must be kept in controlled conditions that mimic their natural habitat for them to survive and thrive.
In short, a kangaroo might be able to visit a polar bear in Alaska for a while but he can't live there.