Most times when I install something it asks to be dragged into applications

I kind of get the general idea from this question Why do I have to drag my new apps into the application folder? that I've just unpacked an archive I downloaded. but I'm wanting to understand what the alternatives for me are and any benefits of dragging the unpacked application to a different "folder" of my choosing. Is there an advantage to creating separate task oriented folders at all?

Trying to get comfortable with my imac, but I keep forgetting that dragging into "applications" does not actually complete the installation. So why are the web download install experiences not just a one click and you are done activity? (I would use the store, but the app store far too often shows me other unrelated apps above the one I am looking for/ wastes time /distracts me too.) I feel I'm missing some obvious flexible decision point?

It's just a convention that adds a layer of security.

Some apps like VirtualBox, for example have an install script that will do this for you, so obviously it can be done. However, if you have to manually do this, there's no way an inadvertent double click will install the App.

Secondly, apps can be installed for both users system wide or for individual users (~/Applications) so this manual step gives you a choice of which Applications folder you wish to install it to.

“Completing the installation” is a misnomer here on macOS. Unlike Program Files on Windows, you don’t have to have a special place to run an app. You can put that app bundle almost anywhere (obviously not protected volumes) and have it work. You're only copying the the app bundle to the Applications folder for convenience.