Objects such as candles help provide a focal point for one's energies and concentration, and that is why they are sometimes used in meditation as well. Other tools are used in ritual settings, and also in spellwork to act as a conduit for one's energy. Sometimes they are also used to help distinguish the difference between the sacred and profane. This subtle difference in the environment or surroundings of a ritual space can mean the difference between everyday occurrences and special circumstances.
Oils are often used to "dress" candles and sigils. I find that a candle burns differently with the use of oil than without. Herbs are often incorporated into candleburning, or used to dress an altar, or are used in other ways (like mojo bags, incense, etc.) Herbs and oils stimulate the brain via olfactory senses, or sense of smell, and also lend energy to spellwork.