Why is there two ways to export a photo?

Dragging a thumbnail from the library to the Desktop will get you the preview version of the file which is often smaller in file size and pixel dimension.

The correct way to get a photo out of the library is to Export either the Unmodified Original or the edited version choosing the quality and size of the image at that time.

AFAIR in older Photos/iPhoto versions dragging to a Finder window did the same as exporting original. I wish it still did the same because the current setting is potentially destructive for a careless user.

Matti Haveri wrote:

AFAIR in older Photos/iPhoto versions dragging to a Finder window did the same as exporting original.

That's not entirely accurate. If the original is a jpeg then it is with the latest Photos version.

However, if the original is an HEIC image file the resulting jpeg will be larger than the original.

Also if the original file is a RAW file the resulted dragged jpeg will be smaller than the original. By smaller I'm referring to the file size, not the image size.