It seems pointless. You’re being asked to document your great-grandmother’s second or third spouse, the one not actually in your line. Why?
Name changes. Some societies will ask for all marriages just to ensure the completeness of the genealogical record. Most will ignore additional marriages for the male but ask for them for the female. The request is driven by the desire to document name changes. Historically, women in the United States have changed their surname on marriage, so the name at death is often the surname used in the last marriage.
Whether or not you need to document the marriage itself will depend on the society. Some will ask for it out of policy; others may not, provided the ancestor’s death certificate lists their parents. (If the death certificate or obituary does not name parents or if the names are too common to clearly indicate the same ancestor, document the marriage!) If you’re trying to stay ahead of any issues, it’s best to get the documentation.