From a video where everyone from the
Victorious cast thinks that
Ariana Grande is
the best singer in the cast but Victoria says that everyone is a good singer. (Maybe because she is jealous of Ariana and didn't want to admit that Ariana is the best singer)
So basically.. You say this when someone says something about themselves, another person etc. and you won't admit that they're right, or something. You could use this in other situations too but i think this is the main reason.
A:
I'm a very good singer..
B: i think we ALL are very good singers!!