In some cities, there's either no organised walking tours, or just one, so there's nothing to choose. Sometimes, you want a specific tour, perhaps architectural or to see movie/TV filming locations, ...
A few weeks ago I was in Berlin, which has a huge number of excellent walking tours, many of which are free (you pay a tip at the end). These covered the usual sights, as well as some more interesting ...
For instance, Sandeman's runs free walking tours all over Europe. One or two every day of the year, good local guides, and the only payment is tips at the end. I'm sure the tip money is good since ...