Symbolism, Imagery, Allegory
Billie Jo's seen fire and she's seen rain, but they represent different things in terms of how she identifies with her environment. Just as the apple trees symbolically reference Ma's life and legacy, rain is always portrayed as a symbol of hope. Just look at Daddy's reaction—the same guy who was digging the giant, scary hole is now "dancing outside in the drenched night" (90.9). Furthermore, even though his cattle have been starved and shot, even "Joe De La Flor / is singing in his saddle again" (91.1) as he lets his remaining cows out to graze.
The rain doesn't just symbolize hope, then, it also symbolizes transformation. People shift when the rain comes, they get temporarily unstuck from the worry they near-constantly carry. You might even say they sort of bloom.
Rain is used as a metaphor for hope in the chapters' titles. Chapter 29, where Ma stands in the rain, is called "Hope in a Drizzle," while Chapter 90 is called simply "Hope" and features a snowstorm that gradually turns into the vigorous rain the land needs. Chapter 92, however, is called "Hope Smothered," as the dust whips through and undoes all the good the rain accomplished. If dust represents hardship, rain represents the hope that someday, something better will emerge from it.