Torah (Shemot 38:8) tells us that the Kiyor was built with the copper mirrors brought by women. The Midrash (brought in Rashi ad loc) explain that, at first, Moshe wanted to reject the gifts, but Hashem told him to take them, because they were great. Why? Because they helped the Bnai Israel to multiply.
So, why take them now? Wasn't better to leave them to the women, so that they could use them again and continue to multiply?
Maybe the women had now gold or silver mirrors. They, indeed, received many treasures from the Egyptian, either by asking them or by the Sea butin.
So, copper mirrors were now obsolete. But the women kept them as an emotional value (of their use in Egypt).
And, when the women had an opportunity to bring these mirrors to an eternal life, they all happily gave them for the construction of the Mishkan.
Personal answer, have no sources.
I think we can answer this question by understanding how the mirrors helped them multiply. As the Rashi you cited states:
כשהיו בעליהן יגיעין מעבודת פרך בשדה היו הולכות ומוליכות להם מאכל ומשתה ומאכילות אותם ונוטלות המראות וכל אחת ואחת רואה עצמה עם בעלה במראה ומשדלתו בדברים אני נאה ממך ומתוך כך מביאות אותם לידי תאוה ונזקקות להם ומתעברות ויולדות שם
When their husbands were worn out from the difficult labor in the field they would go and bring them food and drink, and feed them. And they would take the mirrors, and each one of them would see herself with her husband in the mirror and would seduce him with words: "I am more beautiful than you". And with this they would bring them to a desirous [state] and they would cohabit with them and whey would become pregnant and give birth there.
It seems from this that the mirrors were necessary to stimulate the desire of the men because the men were too worn out from their hard labor to stimulate their own desire. But this was a particular set of circumstances unique to their slavery in Egypt. Once they were free the desire-block that the men had in Egypt would be gone; they would thus presumably desire their wives without the mirror-machinations. Therefore, the mirrors would no longer be necessary in the same way that they had been in Egypt, and it would be fine to donate them for use in the Tabernacle.
The point was that they did not need the mirrors anymore to convince the men to continue to raise families. While in mitzrayim, the men were so downtrodden, overworked, and in despair, that they would have just given up if the women had not taken the initiative. Consider that even Amram was ready to give up and divorced Yocheved. He would not have been willing to have Moshe, had Miriam not convinced him. Once they left mitzrayim and had received the Torah, the people were happy to serve Hashem and raise their families without the external stimulus of the mirrors. The women wanted to donate the mirrors to the Mishkan in thanks for what Hashem had done and to acknowledge what the mirrors had done for the people.
Rashi points out on Vayakhel 38:8 that the כיור was designed to make peace between a man and his wife. Thus, it was appropriate that the women donated in memory of having been used to create the families of those who survived the slavery in Egypt.
This is [the meaning of] what is בְּמַרְאֹת הַצֹבְאֹת [lit., the mirrors of those who set up legions]. From these [the mirrors], the washstand was made, because its purpose was to make peace between a man and his wife.
Rav Hirsch says:
It is deeply significant that the vessel of the Sanctuary which was to represent "the moral keeping holy of ones acts and efforts" קידוש ידים ורגלים was made out of the women's mirrors. Mirrors are articles which lay stress on the physical appearence of people being an object of special consideration. So that it was shown that the physical sensual side of human beings is not merely not excluded from the sphere which is to be sanctified by the Mikdash, but that it is the first and most essential object of this sanctification. After all, at rock bottom, as Man has complete free will in moral matters, it is just this side of human nature which is necessary to come under the influence of the Mikdosh, if the sanctification of life which is aimed at is to be achieved.
Note: Rav Hirsch points out that
the כיור was made up of the mirrors fitted together almost without any alteration at all, so that it was recognizable that the basin consisted actually of mirrors.
