Apps just offer a better experience than a mobile browser, generally speaking (I’m sure there are exceptions though). Also, apps can send push notifications, and third party apps offer more customizations and better features than official apps (again I’m sure there are exceptions though).
Social media sites and their official apps, outside of the fediverse, are too cluttered and annoying. I still remember when Instagram forced video ads to autoplay. I deleted my account and never went back. I really don’t understand how people manage to use this stuff, it’s awful. Ads everywhere, non-chronological timelines, no customization options, tracking scripts/pixels, that slow everything down, it’s terrible.