No if you want to spread the light in front of you for example outside walking
A diffuser can also be useful in an outdoor setting where you can't do a ceiling bounce. . .
" Wouldn't just bouncing the light off the ceiling be brighter?"
Not really, if the cieling is high enough you would just be lighting up the ceiling evenly but very very little light would come down.
Not sure how important it is, but a diffuser will also prevent the light from casting sharp edged shadows and pinpoint reflections which you would otherwise get from the spill part of the beam.
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