The Via De La Plata follows the route an ancient roman road linking Cadiz with the silver mines of Asturias in Northern Spain. It was used mainly by pilgrims from North Africa who travelled from the 9th century onwards to Santiago de Compostella. For this reason it is also called the Camino Mozarabe.
The reason they traveled there was a shepherd, claimed in the 9th century to have seen a vision of a light, shining on a hill, on what would be the site of the present day Santiago cathedral . On hearing this the local archbishop examined the area and it was said he discovered a stone coffin, containing the remains of the Apostle Santiago or St James (The Greater) as he is known in English.
It had been long rumoured that his body had been brought back to Galicia, after his martyrdom. He had formerly ministered in the area, with mixed results. So instead of silver in the hills, Pilgrims it could be said were after gold because St James or Santiago, was believed to be the brother of John the Baptist, and a cousin of Jesus. Touching the tomb of Santiago, was as close as you could get to the Apostles, and the blood line of Jesus !
In medieval times, it was believed a pilgrimage to one of the 3 main pilgrimage sites, Rome, Bethlehem or Santiago, was enough to absolve you in the eyes of God of grievous sins that may have been committed.
The scallop shell then subsequently, became a symbol of pilgrimage, as the pilgrims continued on to Finisterre, then considered the end of the known world and took one as a souvenir and proof of passage.
Archeologists in Ireland, have discovered scallop shells around the necks of Augustinian monks, from the 14c in Mullingar. Some had 2 or more, so clearly it was seen as an important thing to do. A considerable feat in those days and very dangerous. Journeys might take many, many months or years. Facing wars, disease, pestilence not to mention bandits, wolfs, and an unforgiving, hostile landscape. It was accepted that you might not return. Pilgrims came from all over Europe, and its popularity only waned, after the black death epidemics, and the subsequent fear of travel.