I've gone rim-to-rim at the Grand Canyon; but I never even got a t-shirt.
A Compostella is a document issued by the church to a pilgrim. One, must first get a “passport” and then fill it with stamps. At least two every day during your pilgrimage from your start to finish. It is proof that you moved from point A to Santiago and collected stamps at various churches, public buildings, places to stay or eat. It also documents your journey. It prevents people from showing up at the Cathedral and claiming they spend several days walking when they did not perform a pilgrimage.
People have been doing the Camino since the Middle Ages and there are lots of special traditions associated with performing such a pilgrimage.
http://www.theroadtosantiago.com/getting-your-compostelapilgrims-mass.html