I would add that CSF has many diagnostic uses, and is sometimes extracted for analysis. For example, most of what we know about the role of serotonin shortage in depression comes from looking not at brains (which is hard and destructive), but looking at CSF, which we can assay for 5-HIAA, the main metabolite of 5-HT, or serotonin.

There are persistent rumors that LSD (acid) and MDMA (ecstasy) leave crystals or other residues in the CSF, or otherwise harm or deplete it. These are demonstrably false.