what are the advantage of current transformer?

These are used with low range ammeters to measure current in HVAC 's where direct connection of instruments is impratical.
They not only insulate the instruments from HV lines but also step down current in a known ratio. ( i .e . )

Iline =(I1 \I2) * Ammeter Reading

where
Iline = Line Current

I1 \I2 = Current Ratio