Why is por needed?

Hola Shirley

This is a case of not being able to match the translation of both languages completely. 

When we talk about walking the Camino de Santiago, we would actually say in Spanish "hacer el Camino de Santiago", we wouldn't normally say "andar el Camino". 

In this sentence:

Usted anduvo por el Camino de Santiago el año pasado.

(You walked the Camino de Santiago last year.)

what we really mean is "you walked in the area of the Camino de Santiago", so the person used some of the paths that are meant to be part of that route. That por conveys the idea of "around, within an area" without being specific. As this is not possible to transfer to the English as such, the most accurate translation was "to walk the Camino". 

Here is a quick lesson on POR with that meaning. 

I hope it clarified it.

Saludos

Inma