A married Jewish woman generally immerses in order to exit the niddah status. However, she is permitted to immerse in the mikveh at other times for spiritual reasons. For example, some women immerse for purposes of teshuvah (repentance) on erev Yom Kippur and/or on erev Rosh Hashanah, or as a "segulah" before giving birth or when dealing with infertility.
When immersing for spiritual reasons only, a woman does not count seven clean days or complete the usual preparations (chafifah), and thus the immersion does not affect her niddah status. For reasons of cleanliness, it is appropriate to shower beforehand.
A woman immersing for spiritual purposes does not recite a bracha over immersion.
Unmarried women (single, divorced, or widowed) are generally not permitted to immerse in the mikveh at all, even for spiritual purposes. In many communities, single women may immerse on erev Yom Kippur; a local halachic authority should be consulted.
A woman who immerses as part of the conversion process does not count seven clean days. However, she does a complete preparation (chafifah) for the mikveh in the same way as a woman exiting niddah, and she does recite a bracha over immersion.