Because /tell (and /msg and /w) uses a target selector you can exploit it to find out information about players. This is not so relevant in 1.12 because you don't have the nbt target selector, just the distance or coordinates, but in 1.13 you can find out anything about a player(every nbt data) with this.
I am playing a pvp survival game against some other players and want to know if there is someone near me. I just tell everybody in my radius a random text and see who I messaged:
or, in 1.12:
Now I will get some text in my chat for every player who is in radius of 100 meters.
To find out if I should fight him, I check if he has a diamond sword:
This only works in 1.13, because the nbt selector will be added then!
This will again give me a message if he has a diamond sword.
More examples of commands:
Check who is wearing iron helmet:
Check out his enderchest items and find out if he has golden apples:
Find out if he is deep underground and probably mining:
Find nearest player:
Find all players who are in the nether:
The possibility are endless. I would highly advise to make a command that is limited to player names only, so survival players can't exploit target selectors.