When doing long tours you want to replace stuff before they break. For instance, I did several tours between 3000 and 5000 kms. I might have gotten away with not replacing the drive train between tours, but spending that extra cash (especially when on the more budget friendly end, like me) is money well invested.

Also, with heavy load (bike+luggage for me was usually in the 35-40 kg range) and more riding in rain than I would do when not on tour, leads to increased wear.

My worst failure was a cracked rim on a rear wheel that I should have replaced before the tour. It had started to wear down due to bad breaks. It was still rideable for another 100k to the nearest bikeshop (this was in southern Spain).