These altarpieces (Catalan: “Capelletes”) are a sort of public altars containing a religious image and displayed in the street. They can be found in many cities and villages, in religious buildings (churches, monasteries, and brotherhood houses), civic or military premises, or in private properties (either in the facades or in the interior). Their purpose is to evoke the displayed image, to regard as sacred a particular public or private spot, or to invoke protection from a patron saint.
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Source: http://www.youtube.com/watch?v=Ngz1CUyU3QU