Challenge wins determine who controls the endgame and which alliance wins BB.
See Drew\Cowboy, The Friendship, Boogie, Dick\Danielle, Team Christ, and Ryan\Adam.

Comparing Janelle and Sheila is like comparing Terry on Survivor with Scout or Denise. Sheila was brought to F3 because she sucked most at challenges, the people who dragged her were right, and they booted her ass as planned.