Hidden Blessings

“A blessing rests only on something that is hidden from the eye.” [Taanit 5b]

"And they gave their father wine to drink on that night....."[Vayeira 19:33 ]

According to the Zohar, the dot on the word - וּבְקוּמָהּ - alludes to the fact that God was secretly assisting this event, because Moshiach was to materialize from it, since Ruth, King David's great-grandmother, was a convert from the Moabites.

The latter event between Lot and his younger daughter [19:35] is written without the letter vav, to indicate that the union did not produce such great offspring.  Rabbi  Shimon said "When the verses states that Lot wasn't aware, it means he wasn't aware that Moshiach was destined to come from this union."

Why should the beginnings of Moshiach occur in such an undignified manner?

Ramak explains that when a very lofty soul is about to descend into the world, the forces of kelipah [evil] oppose the soul's descent vehemently.  Sometimes, however, kelipah will consent to the soul's descent if it occurs amidst a particularly sinful act.  Thus we find that from this undesirable act the ancestor of Moshiach was born.

Source: Lubavitcher Rebbe