I can't speak for Symantec, but folks use SSLMate (which resells Comodo) because of the customer support, the wildcard certificates, the lack of rate limits, and the central management of certificates (which integrates with Cert Spotter, our Certificate Transparency monitor). Also, some of our customers have special requirements and unfortunately can't automate certificate issuance. These customers want year long certificates with email validation, but still prefer using SSLMate's command line workflow over the clunky web interfaces of most CAs.