I would guess that's it's not primarily about prestige, except perhaps for the top level competitors. A well-known marathon gives a concrete deadline for people to train toward. The fact that thousands of people are running at the same time gives a feeling of solidarity.
Those other runners also show most of the competitors that there are others who aren't (much) faster or (much) more athletic that can complete a marathon, and the spectators may also provide inspiration, if only because one may feel a loser when giving up in front of a crowd, if not entirely exhausted.
