--is designed to allow passive absorption of enough B12 (usually, the absorption rate is 1-2%, so figure on 5-10mcg from a 5000mcg dose) to start to rebuild stores and for availability to be raised enough for tissue use.

These large doses were specifically designed to get around the usual absorptive channels that are often compromised in people who don't have enough stomach acid to break B12 out of food, or who have (the misnamed condition, as it is not an anmeia per se, but can result in anemia) pernicious anemia.