I’ve done that pilgrimage, and from what I know of the shell, it was used as a bowl/eating and drinking container for the pilgrim on their way to Santiago de Compostela. It was just a handy and available item to have along the journey.
The shell was not obtained at SdC, the pilgrim would have used it along the way there and back. The thing usually desired was the official ‘Compostela’, the church document acknowledging the validity of the pilgrimage and obtain the mass. This Compostela was often taken back as proof of the journey and in that absolution of a crime (which the pilgrimage would satisfy as a choice of going to prison).
In modern times, the scalloped shell, with all lines in the shell converging towards a central location symbolized the pilgrims from all over coming to SdC.
And yes there is some evidence of a pagan pilgrimage along that route preexisting the christian one, but to the ocean. SdC may have been a stop, but the goal was to travel as far west as one could to the end of the land (Fisterra/Finesterra. fin = end, terra=land). There one would go though a spiritual rebirth at sunset and the burning of their clothes.
That combined route has also new age meaning of lay lines, spiritual pathways of energy that draw people to them and in that line explains why both pagan and Christian routes are overladed.