Craneflies are only occasionally important. There are rumors of fishable mating flights, but most anglers will not encounter them. The larvae are probably the most important stage in the trout's diet.
Craneflies pupate for one to three weeks. The species which do so in the water then swim to the surface to emerge in the style of caddisflies, and are presumably vulnerable to trout, though I have not read about good fishing during these events.
Swisher and Richards in Selective Trout say the larvae all crawl out of the water to pupate, conflicting with the above account. There are so many species that it seems likely both behaviors occur in some species.
Adult craneflies are occasionally important during their mating flights when their clumsy flying can crash them by accident into the water. Windy days may also blow them in.
Larva & pupa biology
Diet: Mostly vegetation
Many cranefly species live in many different underwater habitats, and others live in moist soil nowhere near the dirt. We anglers are only concerned with the former.
They live as larvae for about a year and sometimes the larvae end up in the drift in good enough numbers to get the trout excited. Buggy grub-like flies are locally known to perform well on certain rivers, and this may be due to a good population of drifting cranefly larvae (or perhaps I speculate too far). In general, larvae are more likely to be important than the adults.
Ernest Schwiebert notes in Matching the Hatch that they are most common in trout stomachs after the water has been high.