Using half of a 240v dual-phase circuit as a 120v single-phase circuit (US)
The problem with doing this is you may have, say, a 30 amp double pole breaker on your 240V line which may be 10g  wire,  and the you come off it on one phase with a 120V circuit with 14g wire, the 30 amp breaker is to high for 14g wire, which then is not protected from melting.

The US has moved to 4 wire 240V service, to accommodate appliances that have internal 120V electronics.