So, presumably what you mean to be asking is, “Why should a new user care if one distro is based on another one?”
The answer to which is: on the whole, you shouldn't care. What's based on what is useful information to an experienced user because it tells you things like what sort of package management to expect, what types of security features are employed, and what level of hardware support is likely. Moving between distros with common upstreams is less jarring than, for example, jumping between something derived from Debian to something downstream from RHEL.
But to a new user it's all going to be foreign, so what distro is upstream of another shouldn't have much impact your choice of which one(s) to try.