I understand there's the type that actually makes a person Tamei, because the person did bad things (e.g. spoke Lashon Hara). But assuming tzaraas stems from a spiritual affliction, what purpose does a Tzaraas that's tahor (e.g. doesn't have all the simanim) have?
A few possibilities off the top of my head:
- The "non-serious" tzaraas may be actual skin afflictions instead. (E.g. Sforno says that the detail described in tzaraas for clothing precludes it from being a bad reaction to faulty dyeing process.)
- "Non-serious" tzaraas would serve as a very strong warning sign for someone, before getting the real deal and needing isolation.
- By listing the types that need isolation and those that don't, the Torah is making very clear that this is a spiritual thing, not a physical disease (as Sforno, Hirsch, and many more explain).
- In the case of head-to-toe tzaraas, there's no need for a commandment to isolate the individual, as people are likely to naturally avoid the person.
- The details are given so we can learn spiritual lessons. I heard from Rabbi Breitowitz (quoting baalei mussar ... maybe R' Dessler?) that the symptoms given in the Torah indicate dangerous defense mechanisms that prevent a person from dealing with a problem: it's-so-bad-I'm-hopeless (tzaraas that spreads); it's-not-so-bad (white hairs inside); and okay-it's-bad-but-so-what-I'm-really-good-inside (healthy skin inside).