I've always been a great believer in walking as a way to understand places. If you've walked to Santiago (or cycled, or even like one disabled lad I met, been taken by car, sleeping in a refuge each night with his helper), you understand something about the place and the pilgrimage that isn't accessible to people who've just arrived at Santiago by air.
The track you take forces you to understand places in particular ways. For instance, it's linear - you see one church one day, and one the next, and you can't help comparing the two. But it's not like comparing pictures in a museum - you're comparing what's in front of you with the memory of what you saw yesterday. You can't go back and take another look.
And also, you start to understand the locality, and something of its history, through what you see. When you've been walking through chestnut forest all day, a stew with chestnuts and mushrooms in the evening seems a natural complement. Or when you've seen three or four roofless castles, you start to wonder when the roofs disappeared.
The pilgrimage routes are obviously focused on this kind of experience. I've walked part of the Via Francigena, from Lucca to San Quirico d'Orcia, following the track of northern pilgrims to Rome, as well as various ways to Santiago. You're following the footsteps of thousands of pilgrims over the years, and that gives the route a particular resonance.
But there are other 'routes' as well, like the Romantische Strasse in Germany, or the Ruta de la Plata in Spain. The Spanish tourist office in fact proposes a number of routes, all focusing on particular epochs of history or types of art - like the Ruta del Califato, which links sites associated with the Moorish kingdoms of the south. There's a Camino de Sefarad too which explores Spanish Jewish culture and history.
And then of course there's the possibility of following an army (Hannibal across the Alps), or a writer (Thomas Coryate to India), or even a mad morris dancer (Shakespeare's comic Will Kemp who danced from London to Norwich). There are even people already re-enacting Patrick Leigh Fermor's 1930s walk from Holland to Istanbul...