Drag and drop may or may not be equivalent to copy/paste. In case of Windows Explorer, their behavior is the same but it could be different for other applications.

E.g. let's say you have a word document, if you drag and drop it on an open Word window, it will open it. You can't do copy/paste to open the document. If you have an FTP client with drag and drop support, you can drop a file to upload it to the FTP server. In general, a program can attach its own meaning to drag and drop which could be different from copy/paste.