Why does a Nazirite need a sin offering?

I am currently working through Numbers 6:1-21 as I put the finishing touches on a paper I’m giving at the MAR SBL later this month. Most of my work on this passage to date has focused on the ritual proscriptions in vv.1-12, but  the actual ritual instructions (תורה) are contained in vv. 13-21.

In vv. 14-15  a list of offerings and materials are listed. While v.15 expalins required grain offerings, what through me for a loop was v. 14:

והקריב את קרבנו ליהוה כבש בן שנתו תמים אחד לעלה וכבשה אחת בת שנתה תמימה לחטאת ואיל אחד תמים לשלמים

6:14 As their offering to YHWH they shall present: one male first-year lamb without blemish as a burnt offering; one female, first-year lamb without blemish as a sin offering; one ram without blemish as a community offering…

A required burnt offering (עלה) and community offering (שלמים)  make sense in the context of this ritual. The burnt offering serves as a general catch-all in Priestly texts; and the community offering helps underly the sense of joy at the completion of the vow. But, why is there a sin offering (חטאת) ?

According to Jacob Milgrom, sin offerings are used to purge the sanctuary from sin. In Priestly circles, the sanctuary is like a sponge that sucks up all the accumulated sins of the people. The sin offering (and the riuals of Yom Kippur) are meant to cleanse the sanctuary of this patina of pollution before the presence of God can no longer abide in the sanctuary. That being the case, how does a Nazirite faithfully completing their vow cause the sanctuary to be defiled?

In a note on Numbers 6:14 in the JPS Numbers Commentary, Milgrom himself answers the question with an appeal to the rabbis:

It is Ramban (followed by Abravanel) who points to the most likely answer: his [the Nazirite’s] self-removal from the sacred to the profane realm requires sacrificial expiation. (48)

In essence, the return of a Nazirite to a quotidian state would send off impurities but without “sin” per se. This seems most akin to what David Wright refers to as “permisable impurites” such as menstration, ejaculation and childbirth — actions that create impurity but are not seen as sinful or bad.