I've been exchanging languages for over a decade.
I exchanged:
Chinese for Japanese
Chinese for English
Chinese for Spanish
Japanese for English
Japanese for Spanish
Here is what happened on my language exchange:
I learned languages. Of course I do. From a "dumb Japanese speaker" to a real Japanese speaker, even found a job from it, paying bills by it. Oh Japanese language is my truly "friend with benefit", HUGE BENEFIT!^^ And I learned English too. See, I can write stuff on Quora now! Yay! My Spanish is still on a "dumb" level, but it's on going, it's such a bliss to feel the distance from me to Latin America-my dreamland shortening day by day by the constant practice.
I learned communications. After 5minutes about "glad to talk with you" "I'm from **" "What's your hobby" etc, how do I break the ice?; What kind of topic is appropriate, safe and not too boring?; Should I correct my partner's pronunciation "danren" into "dangran" when she tries so hard in a conversation?"; Did I say anything bad/taboo/offensive? (Why he/she sounds upset?); My partner is on a different language skill level with me, how do we balance it to make a fair and comfortable exchange? Should I end a conversation when it is really late but she/he is talking excitedly? (If yes, how?) ; How do I deal with people who want a romantic date rather than a language exchange? ... To communicate with people from different background/culture/religion/belief is such a big challenge. And the feelings of "always unpredictable things there" make life EXCITING!
I realized my ignorance. I didn't know there were blue/red/black corn which is also eatable in this world until my Mexican partner told me she loved them; I had thought there was only one English accent called "British accent" in UK until my partner from Peterborough SURPRISED me and eventually I got to know the variety of accents in UK is something comparable with the hundreds of Chinese accents in China. I got so so much new information: There are not only ice-cream vans but ice-cream BOATS on a typical beach in Sydney; For some reason many Mongolian people don't like Chinese people, even there is a phrase in Mongolian literally meaning "you behave like a Chinese" and in fact meaning "you are tricky and mean" which shocked me much; There is a guy named "Donald Trump" who is hated by many many US people (and ALL of my US partners) and is also supported by pretty many other US people as well... For me, the conversation with partners from all over the world is always mind-opening and often mind-blowing.
I made unique and amazing friends. I couldn't find any better way to make friends like this- Similar interests (language learning), deep communication, magnificent diversity. Every time before I travel (couple of months or even years before the travel), if possible, I will try to find some local person to learn some local language and local information/knowledge. This process sometimes can be even more exciting than travel itself and always greatly help me "travel like a local instead of a tourist". Sometimes I'm lucky enough to meet up my local partners, which magically makes my trip no longer a thing of "from somewhere to somewhere" but an adventure "from one human to other human". When I can't afford a long time/distance trip, I have such a convenient alternate- just leave a message to my partner in another side of the planet "Hey wanna talk this weekend?"- I know there is a fantastic journey waiting for me.
Eventually, "language exchange" not only exchanged my languages, but also my experience, views, and part of my life.
P.S. Gosh why my answer sounds like some advertising of language-exchange! Oh no...I swear nobody paid me...Well, I admit, I can't stop answering like this. Because I love it!