The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
This tool is unlike any hatch chart you've ever seen: much better in some ways, worse in others.
Rather than providing the traditional graphics of emergence times, each chart work more like field guide to hatches for a specific time and place. You set the location (continental US/Canada) and time of year, and this hatch chart will show you many of the species you might encounter, with some of the most likely near the top. It shows many species normal hatch charts don't. It won't be as accurate about well-known hatches on specific rivers as a good local fly shop's hatch chart, but it can help with understanding obscure hatches or fishing destinations without local expert charts.
When treated as a starting point and not the final word, these hatch charts are uniquely useful.
Because these charts are based on public and scientific records, they vary in reliability and offer only spotty coverage of a species' real distribution, especially for adults. Many if not most records are for nymphs and larvae.
These charts are created on-the-fly programmatically based on professional and amateur species occurrence records in public databases. They take into account the number of such records to hint at species abundance, the timing of adult collection records at similar latitudes, and the extent to which records for a species were recorded near the state or time-of-year of interest.