Knowing that what you say will be private(ish) and not be kept on historical record (at least not in public -- I assume Snapchat saves everything) can lead one to be more open, honest, and up-front in their communication.
Whereas writing something which will be kept on the public historical record for all to see forever leads to both conscious and subconscious self-censorship and can be misleading when trying to "read back who you have been 10 years ago" because what you wrote may not have been what you were really thinking.
The ephemeral nature of the spoken word and the historical nature of the written word both have their advantages and disadvantages.
Do you honestly believe snapchat actually deletes content? Please.
Who said gossips? "Stories" have and utterly different meaning to me.
Journals stick around. Letters stick around. Notes stick around. They are important. We evolved to write, not just talk, and reading back who you have been 10 years ago, even if it's just a plain entry, can be surprisingly useful to maintain a healthy soul, especially in times when you think things are slipping away.
Take away the option to remember, what are you left with?
And no, I'm not exaggerating. No one scrolls back any more; yesterday deliberately out of interest, and I disagree with this from the bottom of my heart.