In the description of the actual construction of the Mishkan, we are given a careful accounting of the precious metals donated for this national project to bring Hashem’s Presence into the community of Israel, and how these donations were used.

Concerning the copper Basin (Kiyor), however, the donations came from a separate source:
And he made the Basin (KIYOR) of copper and its stand of copper, from the mirrors of the women who crowded at the entrance to the Tent of Meeting (Shemot 38:8).
A number of suggestions are offered to explain this exception (see Nechama Leibowitz’s fine discussion on the different views of our commentaries).

The simplest explanation might be that of Haamek Davar (R. Naftali Tzvi Yehudah Berlin, 1817-1893). Here is the Torah’s first presentation of the Kiyor:
And Hashem spoke to Moshe, saying: “And you shall make a basin (KIYOR) of copper and its stand of copper for washing (L’ROCHTZAH) and you shall place it between the Tent of Meeting and the Altar; and you shall put water into it. And Aharon and his sons shall wash their hands and their feet from it . When they come to the Tent of Meeting they shall wash with water, so that they do not die, or when they approach the Altar to serve, to raise up in smoke a fire-offering to Hashem. And they shall wash their hands and their feet, so that they do not die. And it shall be for them an eternal statute, for him and his offspring for their generations” (Shemot 30:17-21).
The Kiyor had not been mentioned earlier with the other vessels of the Mishkan, says Haamek Davar,
“because all other vessels are for the sanctification of the service. But this utensil, even though it sanctifies the water, nevertheless it is only to sanctify the person for the purpose of doing the service, or to enable him to enter the Sanctuary.”
The Kiyor is essentially a means to an end; the main issue is the washing. In fact, although it is the ideal receptacle for this purpose, it is not indispensable. This explains why the instructions for the Kiyor are given separately, and why the copper for its construction comes from a separate source (see also Sforno, R. Ovadia ben Yaakov Sforno, c. 1470-c.1550).

This washing is not cleansing, or purification, but an act of sanctification. Throughout the Torah, Onkelos translates the verb R-CH-TZ as “wash” (in Aramaic, AS’CHEI); here however, Onkelos translates L’ROCHTZAH as L’KIDDUSH, for sanctification. A kohen is obligated to “sanctify” his hands and feet, even if he was purified by immersion in a mikvah.

Rambam discusses this in “Laws of Entering the Sanctuary”, Chapter 5:
10. It is a mitzvah to sanctify from the waters of the Kiyor, but if one sanctified from one of the [holy] service vessels, this is valid; but profane vessels cannot be used for sanctification. …And one does not sanctify within the Kiyor or within a service vessel, but from them [emphasis added].
16. How was the mitzvah of sanctification done? He places his right hand on top of his right foot and his left hand on top of his left foot, and bends over and sanctifies.
Like all Temple requirements, the sanctification must be done standing, so the Kiyor had spigots from which the water would flow over the kohen’s hands and feet.

Rambam also says (ibid.):
1. It is a positive commandment for a kohen who serves to sanctify his hands and his feet, and afterwards to do the service. …And a kohen who served without sanctifying his hands and feet in the morning is liable for death at the hands of heaven. …And his service is disqualified, whether he is the High Priest or a lay priest.
(See also Sefer HaMitzvot, positive commandment # 24.) A kohen does not need to re-wash his hands and feet during the rest of the day, unless he leaves the Sanctuary, sleeps, urinates, or is otherwise distracted. Nevertheless, even if he remains awake all night, he must “sanctify” his hands and feet the next day.

In defining this mitzvah, Sefer HaChinuch (ascribed to either R. Aharon HaLevi or R. Pinchas HaLevi of Barcelona, mid-13th Century) says (§ 106):
“To wash the hands and the feet at any time of entering the Heichal (the holy inner building in the Temple), as well as one who comes to do the service in the Temple.”
This, says Haamek Davar, is in line with the view of Rambam (and unlike the view of Tosafot in Yoma 5b and Sanhedrin 83a): that one is required to “sanctify” his hands and feet when entering the Heichal, even if it is not for the purpose of doing service. His source is the mishnah in Kelim (1:9) that describes the sanctity of the Heichal, which is only one step lower than that of the Holy of Holies:
“The Heichal, … one did not enter there unless he was washed, hands and feet.”

Accordingly, Haamek Davar explains a later anomaly:
And he placed the Basin (KIYOR) between the Tent of Meeting and the Altar, and there he put water for washing. And Moshe, Aharon and his sons washed their hands and feet from it. When they came to the Tent of Meeting and when they approached the Altar they would wash, as Hashem commanded Moshe (40:30-32).
Since “sanctifying” is required for anyone who enters, even without service, then even Moshe would do so whenever he entered the Tent of Meeting to communicate with Hashem.

Ramban explains that those who enter the Sanctuary are bidden to sanctify the two extremities of the body, the hands and the feet, referring to the highest and lowest part of the human form. The total person, from his basest needs to his loftiest potentials, are devoted to Hashem.

And this is done with a man-made vessel. Over and above Divine Providence, man’s own efforts can transform him into a sanctified being.