Traditional understanding held sacred spaces were the only thing that can temporally/partially reunite Heaven and Earth. These man-made sites were holy of holies where God could be present and sin could be taken away. Non-Christian sites were similar despite the lack of the Fall and uniquess of redemption geography theology. For example, mosques brought Muslim communities to God's messages, Celtic forests were the places where the spirits lived, and the Aztec believed the ruins in Mesoamerica were the cities of forgotten God's who needed to be appeased.
Today people go to sacred places for many reasons. Some go for the traditional reason why others seek another spiritual experience, research the cultural meaning these sites have, or just wish to look at the pretty architecture.
The God In America website has self-guided sacred spaces tours for eight American cities. Atlana, Boston, Chicago, New Orleans, New York, Portland, Santa Fe, and San Francisco all of tour pages. The tours have good information on interesting sites in these cities. I only wish other cities that have a plethora of sacred sites like Washington D.C. had guides.