When I say lower, all I mean is technologically and scientifically less advanced.
But using the word "lower" at all reveals an unexamined assumption of hierarchy and value. The insidious thing about prejudice is that it's frequently unconscious. The British Empire and the missionaries believed they meant well and only wanted to help, but their unconscious assumption of superiority led to enormous harm. Before you start meddling in other people's lives, you need to take a hard, critical look at yourself, at your own assumptions and motives, and get yourself in order. That's what the Prime Directive is for.
But it's the truth, one group of people has phasers and is travelling through space and the less advanced is playing in the sand with some rocks.
And that's an incredibly condescending and demeaning thing to say. Lower technology is not lower intelligence; if anything, people with less technology need to use their intellect more, so we're probably dumber on average than our distant ancestors were. (For instance, people in non-literate societies tend to have extraordinary memories, because they need to. And the average person has gotten far worse at math since the pocket calculator was invented.) Ancient humans achieved extraordinary feats of engineering with just "some rocks," figured out how to domesticate plants into edible or otherwise useful forms, devised social and religious rituals, created remarkable works of art, and so forth.
Sure in a long timeline they'll develop the same technology ( if they happen to survive), but at the moment of comparison, one has advanced and the other has not. Technologically superior, medically superior, and more knowledgeable about the world they live in.
But that's just it -- if you're a human who's never been to Beta Exemplica IV before, you
aren't more knowledgeable about Beta Exemplica IV than the Beta Exemplicans who've lived there all their lives, who have grown up within their society and understand how it works, who understand their own belief systems and traditions that you might profoundly misinterpret because you're filtering them through your own prejudices. If you push changes on them that you think will be improvements, you could end up unbalancing and disrupting their society and causing enormous harm. They understand their own world better than you will, and they are not children but intelligent adults able to make their own decisions. So they're the only ones qualified to decide what changes work for them.
As far as how they could help other species without destroying their culture... I'm not sure. But it seems to me that there's no reason to lie to a primitive species about the existence of other worlds when they run into them.
Now, that's the first thing you've said that makes sense. It's true that new knowledge won't be intrinsically harmful to a society. On the contrary, historically, the most dynamic and vigorous societies have been the ones that traded and interacted with other cultures, that were exposed to new ideas all the time. But the key is that they were free to adopt or reject those ideas as they saw fit, to take charge of interpreting and adapting them to fit their own way of living. That process of syncretism -- adapting new beliefs to reconcile with your existing worldview -- is the natural way that societies respond to new input. For instance, when a society is "converted" to a new religion, they generally don't just throw out everything they believed in favor of the new stuff, but fold the new religion into their own existing beliefs and customs -- like the way Vodoun and Santeria identify Catholic saints with traditional West African divinities and incorporate them into practices and rituals that originated in West African religion. By adapting the new beliefs into their existing social structure, they can incorporate them without their way of life being disrupted. But only they know how to do that.
The ideal example of a culture adopting more advanced technology and not being harmed by it is Europe. For many centuries, Europe was considerably less technologically advanced than China and the Islamic world. But over time, Europe obtained new technologies and knowledge from the East -- the stirrup, the compass, the moldboard plow, gunpowder, the lateen sail, the printing press, plus lots of mathematical and scientific knowledge. The thinkers of the Enlightenment were influenced by elements of Chinese philosophy as well, such as the principle that a ruler's obligation is to serve the people rather than his own self-interest. And Europe used these acquired technological and intellectual advances to become the dominant power on the planet. It survived the influx of new technologies and ideas because nobody forced them on the Europeans; rather, they were free to adopt and embrace them at their own pace and use them according to their own needs. And yet when Europeans then tried to bring their own technology, religion, and values to less technically advanced societies in turn, they did enormous harm, because they tried to control the process themselves rather than letting other cultures make their own choices about how to conduct the interaction.