the reason for this follows along the lines that david yarrow mentions, but the
other reasons may be 1) fires provide a nutrient flush that young growth-- sap-
lings; (which is favored food of many species of fauna)-- thrive upon.
2) the fires cleared away undergrowth that inhibited hunting-- arrows would not
be caught in the tanglewood. 3) as a weapon of war.
midwestern stories, and written records are full of a prairie fire history.
for a further "discussion" of these issues, i recommend the very accessible
works of stephen pyne, _fire in america_; and also ethnobotanical records by
kelly kindscher and charlotte erichsen-browne.
yours, pds