You get better performance when it's used for comp (but not comp-3) items used in arith operations.
To get an idea of how much you might gain per instr execution comple your pgm w/the LIST,NOOFF compiler opts. Code a test by uning SYNC for a comp item and don't use it for another. Create the same arith stmt for each, then check the compile listing and compare the generated assembler instructions.
You can use it for group items, but remember 01 items are SYNCed automatically. Not sure if 77s are.