Soaking tortoises are important because the normal waste product for protein metabolism is ammonia. Tortoises convert this ammonia to uric acid, which is less toxic to the bladder wall and lets them hold water in their bladder for long periods of time.
Tortoises are a desert species, so water conservation is a large part of their physiology as they have adapted to their arid environment. They can recycle water from their bladder into their system when they do not have access to drinking water.
If the uric acid builds up in high enough levels, a situation that might happen if the tortoise is not drinking and staying well hydrated, the uric acids combines with electrolytes like sodium, calcium, and potassium, to form urates. In a well-hydrated tortoise, these urates are secreted normally when urinating. In a dehydrated tortoise that is recycling its urine, the urates are not secreted, and can slowly build up into a bladder stone.
Many people believe that tortoises get all the water they need from their food. This is not true, they need to drink water also, so make sure fresh water is available all day and also periodic soaking is recommended.
Click to read more on Tortoise Poop And Healthy Gut Flora
We still find it difficult to get our PeeWee (125 lbs) to simply drink water. We have not had luck in finding a suitable vessel. He does love a good shower though.