Sure, they have seismographs, but they don't necessarily have full details on all the subsurface geology between you and the epicenter. Wave propagation can be

Sure, they have seismographs, but they don't necessarily have full details on all the subsurface geology between you and the epicenter. Wave propagation can be complicated... depending on what's down there, the waves can intensified, dissipated, reflected... If (e.g.) everyone in a small area reports strong shaking, while those in surrounding areas report less, that can indicate a need for further investigation as to whether the area that experienced strong shaking poses a specific risk.