I needed an external DVD in order to reload the Windows operating system from a recovery disk onto a Netbook computer that didn't have a DVD built-in.
Just why the computer manufacturer supplied the recovery disk on media the computer couldn't read is another question. However an external DVD connected by USB worked a treat - no set up necessary, basically just take it out of the box and plug it in. USB v2 of course is almost essential to get the transfer speed.
As a purchase for a one-off job the Toshiba DVD drive is quite expensive. I gave up burning disks as a way of backing up data long ago in favour of USB sticks and an external hard disk. However, although purchased only out of necessity it has become a useful adjunct, not permanently plugged into the Netbook but brought out occasionally to burn a photo disk for a friend, rip a CD, or play a DVD. It does all of these things with no fuss and more reliably than I've experienced with internal optical drives on a desktop machine before now.
What I would say is that prices vary enormously if you shop around online. If it's just a one-off emergency I'd say go for the cheapest. If you want it for more intensive, everyday use it may be worth paying more.