According to the pilgrim office's page 'what is pilgrimage'
http://www.archicompostela.org/Peregrinos/Inglés/peregrinconcep.htm 'To be a pilgrim is to journey to the Apostle's Tomb in Compostela with a Christian or at least religious motivation, that is to say, "pietatis causa".
. . . The motivation is - since it is a pilgrimage to St. James - to visit the tomb of one of the Apostles of Jesus.' However, if you question those en route or in Santiago, you'll probably find only a small minority are motivated by the tomb itself, and even devout RCs will admit to doubts on whether the relics are genuine.
The church is fully aware that people are motivated by a wide range of feelings (and quite a few who aren't at all sure why they're doing it). In his 1999 pastoral letter (the link isn't in the page quoted above; it's at
http://www.archicompostela.org/web/igle ... al1999.htm ), point 42(!), the archbishop comments on those who "set off without specifically religious motives and travel the route through love of nature or for purely cultural reasons. The spiritual dynamic of the pilgrimage favours the quest for God and the response to His call. Many of those who start detached from any religious feeling experience on the Camino a profound meaning in something they had viewed as a mere pastime when they set off. We welcome them all with open arms . . ."
I've always found it striking that most people seem to get something out of it; I've yet to meet anyone who said it was all a waste of time and they should have stopped at home. (Though of course this is self-filtering, as those who do find it a waste of time will have given up long before they get to Santiago.) And I've also met quite a few who felt that St James helped them along the way, and who found some kind of fulfilment at the end.
Of course, if you're determined to be sceptical, few relics of any sort stand up to much close examination. What little documentation there is is generally based on some oral tradition written down long after the event by people with an obvious 'agenda'. There's precious little evidence, for example, that Peter ever even went to Rome, let alone died there. Did the remains of Mark end up in Venice, or those of Andrew in Scotland? Maybe, maybe not. Who can say? And the medieval obsession with relics led to obvious absurdities, such as the several heads of John the Baptist. Erasmus commented that there were enough 'pieces of the true cross' around to build an entire ship. Was Mary Magdalene in Vezelay or St Maximin, or did she never leave Palestine? Is the head of James in Santiago, or in the cathedral of St James in Jerusalem, or forgotten in some ditch somewhere? What you believe is likely to be based on what you want to believe - 'faith' - rather than because you are persuaded by the evidence.
Although the story of how James is supposed to have ended up in Galicia does stretch credulity, I actually find the idea that he went to Spain reasonably plausible. If you accept that the disciples spread out after the ascension (and it must be said there's not much evidence they did any such thing), surely at least one will have headed west across the Med, and if so why not James? For a Galilean the ports of Phoenicia were close by, so hitching a ride on a Phoenician vessel to a Phoenician port, such as Cartagena, as the tradition has it, also seems plausible to me.