you cannot drop the dragged images anywhere, so it seams to server no perpose, so why do some browsers do this.
Correct, servers no perpose except that when you drag it to another web page in any browser, the image will appear together with the URL to that image. Drag it to a word document and you will get the URL. Guess that a question needs to be posed to the makers of the browsers.
I second Gramps on that.
The main functionality of this is to drag to copy images to other programs from your browser.
However, if you want to disable it (I wouldn't really bother to!), There's a neat trick in Firefox.
In your address bar, type about:config
This will bring up a list of firefox config settings.
Search for nglayout.enable_drag_images and double click on the parameter you see. This will change the value to false. This will disable your ability to drag images in your browser.
PS: Be sure not to play with other parameters unless you know what you're doing or you'll end up with a broken Firefox!