No reason to seal them if they are just going to be removed. Obviously the sealing is to protect them from something very serious. Since they are sealed prior to the trib, it's clear the seal is to protect them during the trib. The entire church goes through the trib.
Do not be stumped by Revelation chapter 14 which is about His Body.
We who belong to Christ all receive the seal when we are Saved.
Nevertheless, God’s firm foundation stands, bearing this seal: “The Lord knows those who are His,” and, “Everyone who calls on the name of the Lord must turn away from iniquity.”
Do not work for the food which perishes, but for the food which endures to eternal life, which the Son of Man will give to you, for on Him the Father, God, has set His seal.”
Now He who establishes us with you in Christ and anointed us is God, who also sealed us and gave us the Spirit in our hearts as a pledge.
In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise, who is given as a pledge of our inheritance, with a view to the redemption of God’s own possession, to the praise of His glory.
Now He who prepared us for this very purpose is God, who gave to us the Spirit as a pledge.
Do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption.
who are protected by the power of God through faith for a salvation ready to be revealed in the last time.