What is the purpose?

okay Wendy -
I am curious why you are asking about this therapeutic intervention treatment ???, as I’ve had MS for 23 years and so I became curious myself!

— I’m a voracious reader of medical journals - SO
here’s the NIH description from a journal article- https://www.ncbi.nlm.nih.gov/books/NBK560566/

Plasmapheresis is a therapeutic intervention that involves extracorporeal removal, return, or exchange of blood plasma or components.[1][2]

Indications
Therapeutic plasmapheresis is used for numerous diseases. The conditions involve the presence of a toxic substance in plasma (e.g., immunoglobulin), which can be filtered. The disorders where therapeutic plasmapheresis can be done are grouped into four categories by the Apheresis Applications Committee of the American Society for Apheresis (ASFA).

Category 2 includes disorders where plasmapheresis can be done as a second-line treatment in addition to the existing standard of care

The alphabetical list of various indications for plasmapheresis, along with their ASFA category, is as follows.

Category 2 users:
Multiple sclerosis

Clinical Significance
Plasmapheresis offers an effective therapy for many acute and chronic diseases. Generally, risks and side effects are mild, most common being transient hypotension and related symptoms.[9][10]

The American Society for Apheresis has done extensive research on plasmapheresis as a therapeutic strategy, and continue to update a comprehensive apheresis guideline using an evidence-based approach to aid physicians caring for these patients.[10]
Outcomes
Plasmapheresis is an intensive care procedure that needs appropriate decisions, specialized skill-based training, close monitoring, and follow-up for best clinical outcomes. It is recommended that a customized dedicated chart must be designed at all centers performing plasmapheresis procedures. This should include a brief history, with established/presumptive diagnosis.

The basis for the procedure, along with patient-specific concerns, must be listed. This chart should also indicate the total number of plasmapheresis sessions planned. The healthcare professionals must clearly discuss the clinical and/or laboratory parameters to monitor the effectiveness of the procedure and enlist criteria for completion/discontinuation. The urgency of the procedure (emergency, urgent, or routine) must be delineated while planning for plasmapheresis. These holistic goals can ensure the effectiveness and reduce complications of plasmapheresis in clinical settings.

Apheresis or plasmapheresis