Feet are considered an erogenous zone because they have a high concentration of nerve endings much like ears, lips, mouths, nipples, areolae, neck, inner thigh, scalp, fingertips, palms, behind the knees, lower back.
What can that mean spiritually? Spiritually it means that many nadis end at the feet, and in other areas mentioned. Or big important nadis. The more energy flows to an area, the more likely it is for them to be easily stimulated.
Some people prefer certain areas due to their psychology, as well as cultural stimuli.
Unsurprisingly, many of those areas are ruled by Venus.
Feet chakra, like Afodo said are more related to the root chakra. They're also very earthy in nature, and deal with grounding. One of the best ways to ground yourself when you can't immediately resort to physical exercise or food, is to stand barefoot (preferably on grass or ground, or at least on the ground floor) and breathe through your feet chakras.
Your sacral chakra may feel more open after working with your feet chakras because more energy can rise up from the Earth into, whereas previous the flow was either blocked or challenged.