For starters, it is an abridged compilation of (roughly) the first 8 episodes of the main show. The movie makes it very clear that this is a compilation by the Japanese title calling it 総集編 and Re: (the next movie is also a compliation called Re:Re: lol), but for whatever reason it was not translated into the English title. Cramming 8 eps into a 90 min film makes the story less impactful and detailed, but that's to be expected. (So the 2nd movie is just gonna be 4 episodes worth of plot??) There's also a new original song as the ending theme. That being saidit does have very little new animation even compared to other compilation movies.

Animes make compliation movies for a lot of reasons - some fans just wanna watch it again in theatre, some people wanna watch(or rewatch) the anime but don't wanna go through all the episodes, etc. There's for sure money reasons, but I wouldn't call it a cash grab because the title (at least in JP) makes it pretty clear this movie is a compilation and popular animes getting compilation movies is nothing out of the norm.

Idk if the two movies are building the foundation for a second season (tbh frieren, also directed by Saito, is more likely to get a 2nd season than bocchi, but also I'd argue bocchi needs Saito less than Frieren). Kamisama lease gimme 2nd season.