if pure water is bad conductor of electricity ,then why is it adviced not to touch electrical appliance with wet hands or feet?

Absolutely pure water is a bad conductor of electricity. However when some ions (liberated when some salts are present like NaCl etc) are present in water it becomes a conductor of electricity. The water present in nature and used for domestic and commercial purposes always contains dissolved salts which make it a conductor of electricity. Therefore it is unadvisable to touch electrical appliances with wet hands.