Why do I have to drag my new apps into the application folder?
An application on a Mac is just a folder full of files with a .app extension. However, macOS hides this fact from you and displays the folder as a file with an icon. Now when you download something it's very uncomfortable to download multiple separate folders (without a download manager). Therefore this "App Folder" has to be put inside a single file somehow. This is  accomplished one of three ways:


Using a .zip file (a container that compresses its contents to save space)
Using a .dmg file (which is a flat file filesystem–as opposed to a hierarchical file system)
Make an installer (which, however, also has to be either in a .zip or .dmg because installers are folders on a mac, too)


Since most apps are self-contained and do not need anything outside of the app folder, an installer is not only overkill but also slower than just distributing the app folder.

You can obviously launch apps from wherever you want; it's only a custom or a recommendation to put them into /Applications as it easier for you to find it again and it works with multiple users.