The diagram shows 3 different kinds of removable media drives, that is, drives for which you insert and remove the object that contains the data. Floppy drives were the first such drive invented. CD-ROM or DVD-ROM drives come as standard equipment on new systems. Tape drives are used mostly for backing up data in large quantity.
All of these drives must be connected to the power supply and to the motherboard. You can imagine how crowded it gets inside the computer case with all of these cables running around. Indeed you can run out of physical space and connections for the devices that you want. Sometimes it just won't all fit!
External devices that connect to the parallel or USB port come in handy when there is no more room inside the case. To switch between devices all you have to do is switch the connection. That's much easier than having to dig around inside the computer case to physically switch out a device!! You don't even have to shut down the computer to switch USB devices. No wonder they are so popular!