Does this version of the game just have more scenes compared to the Unity version?
Yes.
What's been added that's not in the Unity version, or when did this version start getting new stuff added to it?
A lot of extra stuff, too much to list all the differences.
There is short list of generally what is different in the FAQ in post #1. The renpy version has alternate scenes, extra scenes not in the original (even before the "ending"), extra options (such as pull out/creampie, or condom on/off, etc...), and lots of revised dialogue (like the inclusion of more family wording throughout).
I started adding stuff before the first release of the renpy version. So, extra stuff has always been part of this project.
Was it just due to not sizeable enough updates that Ferdafs pushed out that people just wanted more added and was met with a simple "well Renpy can support this, so let's go for it"?
Lot of different reasons, but mostly the answer is, because I could.
Some of it has been there from the start (as extra things I wanted to see), others were added as time went on as requests from people in this thread, or contributions. It mostly comes down to what I'm up for doing with this project. Sometimes, it's just the update content. Sometimes, when I have 3 months between content releases, I'm up for doing more.