They provide a place for you to display objects showing or symbolizing what is important in your life, symbols that serve as windows to larger concepts. At my workshops, we always create an altar and people are free to put whatever they want on it. They put objects on the altar such as jewelry, items from nature, or things that have been meaningful in their life. They place poems or manuscripts they're working on, or objects symbolizing problems in their life. At one workshop, a woman walked up and placed a credit card on the altar, explaining that she was struggling with debt and working to achieve financial stability.
Altars provide a physical, tangible way to honor our struggles and experiences.
A wonderful example is the cemetery in Martha's Vineyard where John Belushi is buried. His grave is marked, not by the typical headstone, but by a beautiful, natural rock. When I walk by there in the summer, I'm always fascinated to see what people leave on his grave. People have used the occasion of his death to create an altar that allows them to meditate on their own individual struggles with addiction. It seems prosaic to associate altars, ritual and spirituality with the trials and tribulations of our daily lives.