Is extranonce sometimes necessary for blocking? Is extranonce ever necessary to find a block, e.g., in cases where exhausting the regular nonce range fails to block? Is this the rationale behind extranonce?
Yes, it's almost always necessary.  The current Bitcoin difficulty is 48,807,487,245, which means (approximately) that if you go through all possible values for the regular 32-bit nonce, your probability of finding a valid proof of work is only 1 in 48,807,487,245.  So the vast majority of times, you will exhaust all possibilities for the regular nonce, at which point you can increment the extraNonce and continue.