You may be extremely good with maps, have an excellent memory for detail, have worked out how to get around.........in fact you can arrange your own tour, except you will so busy with the detail, perhaps looking for free wireless connections for your laptop too for that extra information, you won't have a chance to take photos, let alone have a really good look around. But on a guided tour, someone else is looking after all that stuff.
We booked three tours in Singapore, a single day in Paris, another day in Amsterdam, the Hop On Hop Off bus in London, a cruise down the Thames and Seine and at the National Gallery in London, we hired head phone commentary machines. I could not say that any of them not great. In the case of the gallery, we may not have seen as much art, but we learnt a lot about what we did see.
I used to be dead against coach tours, but after visiting the Northern Territory and using tours, after Thailand and using some tours, after Vietnam being one long tour, and now using tours in Singapore, Europe and England, they are the only way to go. Ok, it cost us $400 for both of us for a day in Paris, but it was well worth it.
You can only spend so long observing street life. Then you want to see other stuffs.