I have this tortoise "Jabuti" (Red-footed tortoise), for like 20 years, his shell was never cleaned because he always lived in the garden with mud. I have read about tortoise care suggesting to clean their shell, but why? Do people tend to be hygienic about their pets?
In time lots of stuff can build up on a tortoise's shell. These can be anything from the environment including sand, debris, dust particles and so on. Some may even harbour unwanted organisms like fungi. Another problem is that these lessen the UV exposure from the sun, and the less frequent the cleanings, the harder it'll be to clean.
You don't have to do all the cleaning manually, in the wild tortoises scrape their shells on some fallen tree branch or rock. You can have these for your tortoise around in the enclosure and if he ever feels the need to then he will use them to scratch himself. On another note whatever fills up in the microscopic pores in your tortoise's shell actually itch the tortoise, so like cats and dogs they do enjoy being "petted". It's something your tortoise can like you for so I still suggest you frequently clean the shell, just be sure to use something soft like a toothbrush and do not press to hard while brushing around.
This website should be of interest, especially:
Both the carapace and the plastron of a tortoise or turtle is comprised of living tissue. It needs oxygen and exposure to sunlight to keep healthy.
The build-up of soils, waxes, oils, or other preparations, is actually harmful to a tortoise's shell. The keratin and bony layers contain millions of microscopic pores, and things such as the products mentioned actually clog the pores, causing problems.