Emerging flies - those that break the surface film and wriggle from their nymphal 'shuck' - make up a significant part of the trout and grayling's diets. Why? Simple I guess - they are vulnerable.
To hatch, the fly must ascend the water column, leaving the safety of it rocky shelter on the river bed. When it makes it to the top and breaks through the surface film, it must squeeze free as it makes its transformation into the dun. In doing so, it leaves itself open to attention. Plus, it can't just fly away- it needs time to allow the wings to unfold, dry and become ready for flight.
When you start to think about it, it's a wonder any of them actually make it. Here are a few examples: