You can't "stamp" an email with the family seal. You can stamp a piece of paper and then fax it.
https://en.wikipedia.org/wiki/Mon_(emblem)
Crests, coats of arms, flags... these things kinda-sorta died out in America. (Indeed: people came to America to escape the Nobility's culture from Europe). But Japan has a lot of families with "samurai" lineage, the old Nobles from 500 years ago... as well as a connection to their families crests, emblems, flags, etc. etc.
Culturally, its important to at least enough of them, to keep those traditions alive. Fax works for that. Email doesn't seem to quite do the family seal/mon justice.
After all, if you've got this stamp that's been in the family for the past 300 years, you're gonna want to keep using it for tradition's sake. (Maybe not the actual 300-year-old family relic, but replications of it on modern rubber).