There are 8 limbs of yoga. The asana practice, what in modern times, we call 'yoga' is only one limb of this multifaceted practice.
Another limb of yoga pertains to breath, called Pranayama. In Hindu, Chinese and Buddhist philosophies, there are channels of energy that are affected by breathing.
Throughout, my yoga practice I have used variants of breathwork or pranayama to intensify my practice, unblock a chakra or two, and to connect with my higher self. Aromatherapy has assisted me in penetrating deeper, taking more control over my moods and general wellbeing.
Not that I have anything against incense or candles - I LOVE them! I just use Essential Oils for different things, and I am more particular about them because their affects seem more drastic on my body and mind. I can look at a candle or see a waft of smoke from a nearby lit incense stick, and feel relaxed through seeing, and perhaps smelling these scents. Diffusing, wearing and misting essential oils, to me, is more of a mood; a charm for the energies I would like to feel, a lifting when i want to feel up, and a softness when I need to let things go.
In many ways, my yoga practice moderates my mood as well. Some days I struggle to make it to my mat, but i always know that I will leave feeling better.
Yoga and aromatherapy together - whether in asana or pranayama practice boost each other up and create a more dynamic, interscentual experience.