Just be careful that you don't have the phone connected to the computer through a hub and try to drop 500 songs at once. USB tends to fail with huge data transfers, and hubs make it worse. But whether you use an app or just drag & drop (which is actually using a program on your computer) makes no difference - the same file will show up on the phone.
The only problem you may run into is a song or video that's in a file format that your computer has a program to run but your phone doesn't. Then you either convert it on the computer and copy the converted file to the phone, or find an app for the phone that plays that kind of file. (For instance, you may not have an app on your phone that will play FLAC files. If that happens, or you're on 4.4.2, try
Neutron. It not only does FLAC [which 4.4.2 does natively, but there's a bug in the codec], it uses its own codec. And it's free.)