I actually don’t know why this pattern is so effective, but who cares. The BWO Sparkle Dun is a fantastic fly throughout the entire hatch cycle, and it really is fishable in any hatch of the olives. Being the Baetis or the BWO they work great. Fish probably see BWO Sparkle Dun as an emerger.
Tups Indispensable is a classic dryfly that still works on most rivers, especially late in the season when the small BWOs are hatching. I find the Tups Indispensable particularly efficient when the Ephemerellas (Ignitas and Mucronatas) are on the river.