I use these kind of sites quite often. I just find them more convenient. It's also way easier to instruct a layman to use a website like that than tell them to use the command line tool.

sometimes I diff content that is just part of local files, or a local and remote file etc.. I don't want to clean-up to files for a diff to be readable.

Having a simple web-based copy-paste diff-tool is extremely valuable to me!