Chromoly steel inserts for the pedal spindle to thread into. No matter how good the aluminum, there is a lot of stress at the union of the spindle and cranks. Big guys (like Ed, my son, etc.) landing hard can conceiveably trash pedal holes in cranks - - especially if they've unknowingly come loose even slightly. Plus, with the aluminum threads being more fragile, they are more susceptible to cross-threading and otherwise getting buggered.

My RaceFace cranks on the Yeti don't have inserts, which worried me at first; but so far my fears have not been borne out.