The purpose of the `gathering together to be caught up,` is so that we are joined to the Lord and taken to His own throne in the highest. From there we will rule and reign with Him throughout the trib, and beyond, judging the world system and fallen angels.
Let's assume that when Jesus talked about one being taken and one left that the ones taken refer to those who will be taken up to meet Christ in the air when He comes from heaven (1 Thessalonians 4:14-17). What does Jesus say happens to those who are left?
Luke 17:26 And as it was in the days of Noah, so it will be also in the days of the Son of Man: 27 They ate, they drank, they married wives, they were given in marriage, until the day that Noah entered the ark, and
the flood came and destroyed them all. 28 Likewise as it was also in the days of Lot: They ate, they drank, they bought, they sold, they planted, they built; 29 but on the day that Lot went out of Sodom
it rained fire and brimstone from heaven and destroyed them all. 30
Even so will it be in the day when the Son of Man is revealed. 31 “In that day, he who is on the housetop, and his goods
are in the house, let him not come down to take them away. And likewise the one who is in the field, let him not turn back. 32
Remember Lot’s wife. 33 Whoever seeks to save his life will lose it, and whoever loses his life will preserve it. 34 I tell you, in that night there will be two
men in one bed: the one will be taken and
the other will be left. 35 Two
women will be grinding together: the one will be taken and
the other left. 36 Two
men will be in the field: the one will be taken and
the other left.” 37 And they answered and said to Him, “Where, Lord?” So He said to them, “
Wherever the body is, there the eagles will be gathered together.”
According to Jesus, those who are left will be killed just as those who were left outside the ark were killed in Noah's day and those who were left in Sodom were killed in Lot's day. They will become bird food, according to Jesus.