Nembutal is currently the drug of choice for human euthanasia in countries where that is legal, making it an unsurprising choice for people wishing to end their own lives in other countries. Given it is highly effective, its use is tightly controlled, and thus it is extremely difficult to get hold of at all, and almost impossible to obtain legally. One of the reasons why many people resort to less effective drugs.
Nembutal's FDA approved human uses include treatment of seizures and preoperative (and other) sedation. It is used by vets as an anaesthetic, and for animal euthanasia.