From the introduction:
FlexTale is an attempt to make roleplaying game adventure content dynamic and interesting. A module created using FlexTale is different each time it is run. Treasure, monsters, plot activities, NPC backstories and character arcs, and various actions and plot elements can all occur differently.
Differences can occur using the roll of dice, or the GM can simply choose the content that s/he prefers or feels would be most engaging to the play group.
There are two ways that FlexTale can help make an adventure dynamic: Dynamic Content and Dynamic Plots.