Because you want your landing page to load instantly and React or any other JS-generated site just can't compete in loading speed with a static HTML page.

Uhm, probably not the best idea to mix your saas application, with your business website. I would definitely not recommend that.

And i will also echo the other comments, saying that this is definitely overkill. If you're doing a single landing page, jsut go with static html/css. If you need some content management, then either go with a fully fledged CMS or do the hipster thing and compile into static site.

Because you need a bit of CSS knowledge and to know how intersection observers have to be configured and then everything works with few lines of JS and CSS ... it's lightweight and SEO friendly and not bloated as if you use REACT.

IMO landing pages and SaaS applications should be different projekts/repos