I think that the Gateway Bible commentary on these verses is interesting:
"John's list does not match exactly any of the traditional lists of the tribes of Israel (for example, Gen 35:23-26; 49:1-28; Deut 33:6-25), either in the names or in the order of the names. Most conspicuously, it is a messianic or distinctly Christian list in that it begins with Judah, the ancestor of David and of Jesus, the "Root of David" (5:5). Just as the elder's voice had announced earlier "the Lion of the tribe of Judah" (5:5), so now John heard a list of tribes announced beginning with the tribe of Judah. In each case, however, what John immediately saw was something quite different from what was announced. Instead of "the Lion of the tribe of Judah" he had seen "a Lamb, looking as if it had been slain" (5:6), and now instead of 144,000 from all the tribes of Israel he sees a great multitude that no one could count, from every nation, tribe, people and language, standing before the throne and in front of the Lamb (v. 9).
In each case John's vision accomplishes a transformation (compare Gundry 1987:260). A Lion is transformed into a Lamb, and 144,000 Jews are transformed into an innumerable multitude from every nation on earth! The great multitude, wearing white robes and . . . holding palm branches in their hands (v. 9), break out in worship of God and the Lamb in a manner recalling chapters 4-5 (v. 10) and are answered by the amen of all the angels . . . standing around the throne and around the elders and the four living creatures (vv. 11-12; compare 5:11-14). At this point, one of the elders (also familiar from chaps. 4-5) asks John, "These in white robes--who are they, and where did they come from?" (v. 13). When John disclaims any knowledge of who they are (v. 14), the elder answers his own question: "These are they who have come out of the great tribulation; they have washed their robes and made them white in the blood of the Lamb" (v. 14).....
The effect of the elder's initiative is to assure John's readers that the elder's explanation of the innumerable multitude comes from God and can be trusted. The explanation includes both the "prehistory" of the group John sees (v. 14) and a glimpse of what is in store for them (vv. 15-17). The prehistory is familiar to John from 6:9-11, where "the souls of those who had been slain because of the word of God and the testimony they had maintained" (6:9) were told to "wait a little longer" until their full number was complete (6:11). These "souls" have come out of the great tribulation by martyrdom, and now the implication is that their number is complete. They can be recognized as the same group by the white robes they have been given (7:9; compare 6:11)--like the white garments of the twenty-four elders in heaven. The identification of the souls under the altar with the innumerable multitude before the throne of God testifies to the continuity John sees between the fifth and the sixth seals.
If this is so, it is important to assure the churches of the vindication of those who are (or will be) martyred. So the elder's explanation continues. The martyrs' vindication, he points out, consists partly of what John has just seen--that they are before the throne of God and serve him day and night in his temple (v. 15)--and partly of what he has not yet seen (vv. 15-17). The conclusion to the sixth seal provides a glimpse of the final blessedness of God's people. God will spread his tent over them (v. 15). They will never be hungry or thirsty again, and they will be protected from the scorching heat of the sun (v. 16; compare Is 49:10). The Lamb will become their shepherd, leading them to springs of living water (compare 21:6), and God will wipe away every tear from their eyes (v. 17; compare 21:4)"
