Shells and yellow arrows typically guide you along the El Camino. You will see them on wooden and granite signposts, embedded in the sidewalk, painted on houses, stores, and churches... Many larger towns have designed their own unique shells. Some individuals have them on their home or their wall/fence around their home if they live on the Camino.
Pilgrims are always watching for a sign that they are on the right path. Especially walking through large cities, we never wanted to go more than a block without seeing a marker. It actually consumed a lot of time and energy.
There was one group of men we met up with several times who walked miles and miles out of their way one day because they got on the wrong path. You always wanted to see the signs for yourself - you didn't quite believe the groups ahead of you, especially in the dark. People were quite attentive and always called out to us if they felt we had made a wrong choice. Fortunately we never made any sizable poor choices, and never had to backtrack.
When I Googled "Why is the scalloped shell the symbol of the El Camino?", 603,000 answers popped up. So, there are many stories, myths, and traditions dating back 2,000 years. You will have to research it and decide for yourself.
Many Pilgrims buy a shell early on their journey and tie it to their backpack. Here's mine...a very fun souvenir! It went with me most of the way....