World-building. Establishing the relationship with Riku more firmly (something that'll be important when we get to adapting V5), showing the depth of the bond between Umi and Maki in all its sweetness and spice (). Those sorta things.
There are also some significant changes from the LN. Maki's...
If you fiddle with settings on the site, you can make everything fit to window size regardless of the original dimensions. I know I prefer using that to some AI upscaler images and the weird artifacts that accompany them.
I really don't wanna give a link (forum rules discourage such things), but i will say a Google search for the series' romanji title will leave you two clicks away.
In the earliest parts of the story, definitely. But Keita has grown and is constantly being encouraged to grow even further (mostly by Ruri, but the others are getting better about it). His choices now have sway. He's taking more initiative than his first-doujin version could ever have imagined...
Of what? These characters are just beginning to confront what life in this lifestyle looks like. And Keita's dad hasn't even been touched upon directly.