I used the Vivitar 285 and 285HV models extensively for over 10 years,and have found that the color-coded AUTO modes worked very well in many fast-breaking situations covering events,both news, features, and weddings, and all types of indoor social photography. Setting the flash to a manual fractional power, like 1/2 or 1/4 power works quite well in many situations, but when camera-to-subject distances change rapidly,or when using bounce flash, the 285 allows you to get the exposure "right" about 95% of the time,with one shot in AUTO mode, with no need for a test shot or two or three. When used on-camera, the AUTO mode works very well on the 285.
Using AUTO modes, you can press the test button and get a sufficient light confirmation indication on longer-distance shots, or on shots where you need to bounce the flash a long ways. The 285's AUTO-modes work quite well,and use only as much flash power as the sensor determines is needed. Full-Manual ALWAYS dumps an entire capacitor charge worth of flash,and kills the batteries much faster than using an appropriate AUTO mode for the f/stop and ISO in use, and so those are some of the reasons you might wish to use an AUTO mode. Another scenario is when you need really quick recycling at longer ranges, such as a news event where somebody is going to be present at X location for say, 5 to 8 seconds,and you need a long-distance flash shot and you want to get three frames off in that time; in Manual at full-power, that will not happen, but at the widest aperture at ISO 800 or so, you will get extremely fast recyling due to the Auto Thyristor using only as much flash power as needed. Running the unit off of an external battery like a Quantum Battery 1 or Quantum Turbo will allow you to fire rapid-fire in AUTO mode if needed,much more-rapidly than full power manual will do,and with automatic exposure trimming as subject-to-camera distances change.
When bouncing the flash off of a wall or in a fixed-distance setup using an umbrella, it makes sense to set the flash power to a manual setting,which is very repeatable. If a particular umbrella set-up needs the flash at Full power manual or half or 1/4 power, then it's perfectly reasonable to set it that way. When using more than one flash, manual setting of each unit is the easiest,surest way to make sure you get the light output you need and want.