I am a Linux user, but I don't really know how most things work, even after years of casual use on my Main, I just started getting into Devuan and wondered then, what exacly does systemd do that most distros have it? What even is init freedom? And why should I care?
As for why people don't like systemd, it follows the kitchen-sink approach to software and does a lot of things at once.
For people new to Linux I just want to point out - for better or for worse this goes against the Unix philosophy.
Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.