To me, an altar is a visual embodiment of the awakened mind--life right here and now. The Buddha symbolizes our own stainless mind, unperturbed even in the thickest and most chaotic of circumstances. It's a way of making the ordinary sacred, of reminding ourselves that every moment is it; that there is nowhere we need to go, no state that we need to attain.
It's always right here in front of us. As us.
In a sense, an altar is redundant: we are attending to a physical symbol of our lives with, and within, our lives. It's like building a roof on top of a roof. If everything is sacred, how can one thing be more sacred than another?
Zen practice frees us to wander like the Taoist sage and Ch'an master in Chinese lore, unencumbered by ritual, temple, and self-reflection. This wandering can be literal and symbolic. It can take place on the road, at work, or in a monastery.
It can take the form of changing a car tire, filing TPS reports, and polishing the Buddha on the altar. For in the end, these are all expressions of Buddha being Buddha. The physical altar is a reminder that our lives, and everything in them, are the real altars.