Why does the Torah outline many different degrees of impurity טומאה with different purification methods?
You have corpse-impurity which requires sprinkling with the water of the red heifer during a seven-day waiting period. Beyond that, there are multiple levels of impurity which require possibly waiting some amount of time and then immersion in a mikveh (or immersion in the mikveh and then waiting some amount of time) for purification.
Why is there a need for all of these different types of ritual impurity and purification methods? What is the difference between them that causes them to be treated differently?
The easier the diffusion of uncleanliness is, the more difficult and
the more retarded is its purification. Most easily is uncleanliness
communicated by the dead body to those who are under the same roof,
especially to relatives. The purification can only be completed by
means of the ashes of the red heifer, however scarce it may be, and
only in seven days (Num. xix. 11). The uncleanness caused by a woman
having running issue or during her separation is more frequent than
that caused by contact with unclean objects: seven days are therefore
required for their purification (Lev. xv. 19, 28), whilst those that
touch them are only unclean one day (ibid. vii. 18). Males or females
that are unclean through running issue, and a woman after childbirth,
must in addition bring a sacrifice, because their uncleanness occurs
less frequently than that of women in their separation. (Friedlander translation)