CloudFlare strips HTTPS protection off on their machines and views all plain text. So, you get HTTPS protection on the public Internet in exchange for CloudFlare getting to see everything.
This may or may not be what you want/need.
This may or may not be what you want/need.
The operating mode described by the comment to which I was responding, that of CloudFlare performing all steps of SSL termination, is one operating mode available. CloudFlare offers at least one other operating mode in which they are not responsible for all aspects of SSL termination. In particular, they offer an operating mode in which they do not hold private keys. This is referred to as "Keyless SSL". Thus the concern voiced by the comment to which I was responding, that of CloudFlare stripping SSL, is but one available option rather than the only available option.
Clearer?
And sometimes modifying is a desirable feature.