I use Facebook in a browser on my phone. I don’t understand why, if you can easily use it in a browser, you would need the App?
Sharon, that’s an interesting point. You certainly can use it in the browser, and Facebook would work pretty much as normal. But some things would not. Anything that requires Flash, for example, is not supported in an Android browser. If it requires Flash to run, most developers have created an App to make it work. This is why Candy Crush and Farmville won’t work in a browser, but can still be played on a phone or tablet by using the App.
So do you HAVE to use the App version? No, but keep in mind that not all features will work in a browser version. The App version is specifically designed and enhanced to work on your device, so that is why they are so popular.