In a perfect world, bathroom exhaust fans work well to remove moisture and those inevitable organic odors from the room and from the house. However, some builders of older houses vented such fans into the attic rather than to the outside, and a lot of vent fans are the lowest-cost, “builder” fans that can barely move any air. Most houses have enough air leaks that having an open window nearby isn’t necessary, but if you are in a brand-new, super air-tight, energy-saving house, that could be a problem.
I much prefer there to be both an exhaust fan and an openable window in our bathrooms, but cost and lack of common sense on the part of builders has meant that in my first house I had to argue with the builder (and pay extra) to get both in each bathroom, and in my second house, one bathroom has no window at all while the other two have windows that don’t open, but all (fortunately) have exhaust fans that really do get the moisture and the odors out of the room and through the roof.
BTW a lot of those “stink vents” are necessary to make sure your toilets and drains work. They aren’t really for letting odors out.
Also, if you do have a bathroom fan that vents to the attic, make sure to check that the moisture isn’t building up and causing other, serious problems with rot.