Many times people can do the research, know all the routes, but locals (whom we have in spades on SP) know something jarring about local conditions. For example, a forest fire may have recently closed off an area (Signal Peak, UT, summer 2005); we may have a very active avalanche season (Charleston NV, 2005); fires and landslides may have wiped out a trail (Porter and Sentinel CA by the east side); a flash flood may have wiped out the road (Troy Peak NV, 2007); cheat grass seed heads require desert gaiters (lots of desert peaks in 2005).
I've encountered a lot of people who want to snowshoe the North Loop Trail to Charleston in winter. Unless you search the forums here, you could do all the research on Charleston, and still get in a dangerous fix (after all, what could be wrong with a class 1 trail?).