A shunt inserted into the brain to relieve the pressure from Normal Pressure Hydrocephalus drains into the peritoneal cavity. Why could it not simple drain into an external container?
When you lose fluid from your body and if you don't replace the constituents, you'll be deficient in those. CSF when drains into the peritoneal cavity, it gets absorbed into body circulation. But when it drains externally, it's like constantly vomiting or having constant diarrhoea. You become dehydrated, you lose the electrolytes etc.