The wildcard DNS SPF record is useful for mail receivers who won’t check DMARC, but will check SPF records. Also, the DMARC sp= setting defaults to the same as

The wildcard DNS SPF record is useful for mail receivers who won’t check DMARC, but will check SPF records. Also, the DMARC sp= setting defaults to the same as the p= setting (unless the DMARC record is itselt on a subdomain, in which case the sp= setting is ignored). So if you already have a strict p=reject setting, the sp= setting is useless.