This has been discussed in other threads -- flexibility of using horn buttons for daytime but switching to silver for more formal wear of the same jacket. Also allows you to remove buttons prior to dry cleaning, which may be the more common purpose.

Just noticed that Highland Xpress sells the little cotter pins that are required for the process! They call them "button bolts" which is a new term to me but pic and description are exactly what we've talked about.

Wanted to post this in searchable terms so that the next person who has the question might find the answer.

LINK to their Patterns/supplies page or, in case that changes over time, here's their home page LINK