where can I use this expression?
If you ask a person to do something for you, but this person neglects you or doesn't do it.
Than you ask him/her: Why don't you do that for me? I ask you nicely (friendly)?
I think this is a way someone van use this sentence.
I asked you nicely is a common phrase used by people who have alternative ways of making someone do something.
It can be a warning that things may become official or unpleasant or nasty very quickly.