Snapchat is the only social media that interests me. In my opinion the others are shit.
What makes Snapchat different is that it is designed simply to augment already existing communication and interaction with friends. Posts you make can be targeted to your "story", visible to everyone or to custom friends you choose, or they can be targeted at individual friends or groups of friends that you set up; posts are removed after 24 hours and once a friend views a post of yours twice, it vanishes for them and they can no longer view it. This aspect of deletion of information is very nice, it makes the experience feel more like real life, where we rely on our memories rather than an infinite archive of everything we have ever done. But with Snapchat allowing posts to linger for 24 hours, it also finds a nice balance between immediate loss/transition of the experience into memory and archiving of the experience forever.
But the main thing that is nice about Snapchat is how instantaneous the exchanges are. You get so much information in real-time about what your friends are doing, for example if one of them enters an individual chat or group chat then a little avatar of their Bitmoji character pops up and stays popped up so long as they are active in that window. This lets you know they are, at that very moment, looking at the same thing you are. You also receive notifications whenever a friend is typing something or has posted something. They receive these notifications for you too, of course, and in addition you can allow your location to be mapped onto a map and then you can simply click on the map and see where all of your friends are, as well as what they are doing (if they are asleep then their avatar will be sleeping, if they are awake then it will be awake, etc.). You can also customize different actions for your avatar to do.
You can also create custom interactions with you and friends in the Bitmoji avatar thing, and post these as memes or imprint them as a 3D animation as augmented reality into an image of your current surroundings. Snapchat makes it incredibly easy and fast to record video or take a photo, then apply text or graphics or filters to this to create memes. It also links to image searches to pull up gifs or memes by topic you search for. And in addition to text chatting or posting photos, videos or memes, you can also call a friend on the phone via Snapchat, or you can do a Skype/Facetime call with them.
Also you can screenshot something a friend posted, to save it since it will be removed automatically after 24 hours or after you view it two times, but then your friend will get a notification that you have taken a screenshot of their post. I actually think this constant dynamic exchange of real-time information is very cool, because you can apply it only to the people you want, to close friends, and you can still customize it a bit; but mostly it creates a closer bond over space, you feel connected to others in a way no other social media achieves.
The entire purpose of Snapchat isn't to post long dialogues, or to meme-spam, or to post narcissistic selfies, or to search links or quotes or anything like that; the purpose is simply to create a new level of instant connection between you and your chosen friends, a level of connection that does not deny real life connection but actually builds upon this in a natural way. So in a way Snapchat has no real "purpose" in an explicit sense, it isn't about building up huge number of followers or anything like that, it isn't about archiving your content or sharing this content with as many people as possible, and it isn't even about searching for any kind of content at all. It is simply a natural extension of already existing good relationships that you have.