I've never understood why a poster would put another poster on ignore.
It means they can't read or respond to another posters posts. The poster who has been ignored then has a free reign to disagree and discredit the other persons posts, knowing they won't be challenged by that person.
A former poster put me on ignore once for a short period as my posts were so damaging for Bamber. She spent the whole time telling the forum she had done this, and replying to my posts when other posters quoted them. So was obviously itching to read my posts and respond. The poster had just put me on ignore to deflect attention onto me as an undermining attempt.