Why setup ssmtp when using external smtp relay inside rails?

The only reason I can think you might want to use ssmtp is if you cron/other jobs running on the load machine and you want to relay those messages somewhere else. Other that that, if you're using something like sendgrid, why wouldnt' you just use :smtp deliver mode? Even in that case, it doesn't make sense to use :sendmail inside the rails app to me, you should just use :smtp.

Any ideas?