The Lotus Sutra describes how a precious Stupa arises wherever the Bodhisattva Vows are taught and practiced. Inside the Stupa is a Buddha, who voices his appreciation and encouragement. Building a Stupa is an expression of devotion and an excellent way to make merit. It shows our sincerity to practice for the sake of all living things.
"Originally a stupa was a monument over the mortal remains of Shakyamuni Buddha or other holy persons. However, stupas also served as a reminder of other decisive events in the life of Shakyamuni, thus there are stupas at Lumbini, Bodhgaya, Kusinagara, Sarnath, etc. Not every stupa contains relics in the proper sense, in their place sacred texts and representations are also enshrined, which confer their sacredness to the stupa. It is not however, the contents of the stupa that are venerated, rather, the stupa itself serves as support for meditation and a reminder of the awakened state of mind." (from the Shambhala Dictionary of Buddhism and Zen)
In Asia, stupas have been built at the Four Pilgrimage Sites of the Buddha's lifetime, as noted above, and this tradition has been carried to other countries wherever Buddhism has spread. They appear in Sri Lanka, Burma, Thailand, Vietnam, and in the Far East as pagodas, an adaptation of the basic concept of the stupa. It is traditional for a temple to have three features which set it apart as a temple and place of practice: a large Buddha statue; a Bodhi Tree; and a Stupa. At Pine Mountain Temple, the climate does not allow for the growing of a Bodhi Tree, ficus religiosa, which requires a mild climate; however, we have cottonwoods, the leaves of which resemble those of a Bodhi Tree.