Why do Catholic use incense and candles?
Anything you may find used in worship in the New Testament, especially in the book of Revelation, is used in Catholic worship: vestments, lights, incense, chants, processions, and so on.
Candles have the specific symbolism of shedding light, which is both practical and mystical in its meaning. The burning of costly beeswax is also a kind of small sacrifice, and the beautiful Easter proclamation sung over the Paschal candle at the Vigil of Easter expounds. Incense represents the sweet fragrance of prayer and of right teaching, and it also symbolizes purity of heart, since it cleanses the air of any foul odors.
Those Protestants who use none of these things in their worship tend to associate the style of worship depicted in the New and Old Testaments as pagan. This is a strange attitude, but one comes across it. I remember a Protestant pastor's wife who described the Catholic use of incense as "heathen." When I mentioned that it is used in the book of Revelation, she did not know what to say.