Visual Studio option /RTCc - what is its purpose?

Why does it exist?
Documentation says that it “Reports when a value is assigned to a smaller data type and results in a data loss.”
Except it is not what it actually does.
This runtime check reports a failure if discarded by a cast top bits are not the same (all 0 or all 1).
It is not a useful range check for either signed or unsigned types, almost as if someone did it to offend both equally...
I just can't understand why such an utterly useless option has been kept in a compiler for decades.
Am I missing something here?

https://redd.it/1pj0hyr
@r_cpp