To what end pictograms? When and why are they used instead of words?

At a trash can in a city park. The pictogram is a visual, cultural catch-22: you must know what a trash can is and how it is used before the pictogram is helpful. And how helpful is it, really? The trash can may be recognized by form alone; once recognized, the purpose is already known. Does the pictogram make the trash can friendlier? That would be a lot of blank space if nothing were there to decorate it.

Consider also the limits of pictograms; contrast the garbage can with the no parking sign. How do you show ‘no parking’ through a linguistically- and culturally-non-specific pictogram?