The types of apps you're referring to seem to be apps created by internet sites. They are but one class of apps. As to why you might want to use them, well, often, the regular website doesn't work terribly well on a tablet or phone. The app is optimized for the device. Some websites use technology such as Flash (not as common any more) that won't work at all on an iPad. I like the New York Times app rather than the nytime.com site because the layout makes the print easier to read. I don't have to constantly zoom in and out. But, it's really personal preferences. My favorite streaming music sites, Pandora and RadioParadise don't work from the browser. And, in the case of RadioParadise, the app offers a caching feature that lets me keep listening while I have no data connection.
There are also lots of other sorts of apps: games, productivity apps, cookbooks, stargazing apps. Take a look at what's available in the App Store. You might find some things that interest you. Or not.
There are plenty of people who never add additional apps to their iPads, who just use the included apps. Just as I'm sure that there are people who don't use anything on their computers but the included browser. It's personal preference. I find that I get more enjoyment and use out of my iPad with additional apps but you may not.
Best of luck.