1) If you print colour, you need to work in total darkness. The developing tube is light tight, but permits you to fill and empty the chemicals without opening it. This allows you to load the paper in the dark, and then do the developing with the lights on; and
2) It is easier to do a couple of quick prints with the tube, because it isn't necessary to set up and take down as much stuff. It is a bit of a pain though to do much more than a print or two, because you need to thoroughly rinse and dry the tube before each print. I solve that problem (when I am using the tubes) by having a number of them, plus space to air dry them between rounds.
If you use the tubes, you lose the chance to watch the print appearing in front of you. It isn't quite as magical as tray development.