Regardless of whether or not you provide your own SSL certificates, cloudflare still uses their own between their servers and client browsers. So any SSL encrypted traffic is unencrypted at their end before being re-encrypted with your certificate. How can such an entity be trusted?
Cloudflares Web Application Firewall or ‘WAF’ is a reverse proxy that sits in front of your server issuing it’s own certs valid for your domain (cloudflare is a CA, and has control over your DNS to get others to issue certs for them). They then provide caching alongside DDOS protection, geoblocking, various customizable firewall settings, as well as just masking your servers ip with their own. This is their primary service aside from just basic DNS/registrar services.