What is a single pole 30 amp breaker used for
Of course it is OK to use. What you use it for is the question.

You CANNOT user it for general purpose 15 &amp; 20 amp receptacles.

Basically, about the only things you can use one for in a residence would be for a TT130 120V/30A travel trailer receptacle circuit.

Problem is, we see them installed quite often for 20A circuits that are overloaded and tripping. People with no clue about code or safety replace 20A breakers with them.