Because there’s more to HTTPs than just monitoring for certificate expiration dates.
Next to SSL Certificate Expirations, Oh Dear! also scans for Mixed Content, Revoked (Intermediate) Certificates, the use of bad or insecure ciphers, etc.
Knowing that this service is built by Dries Vints, Freek Van der Herten, and Mattias Geniar tells me that this app will be ace!