The main use is to detect how much endotoxins (proteins that cause our immune system to react) are present in a sample. This is important because we often use bacteria/fungus/yeast to produce medicine and then remove the microorganism from that medicine. This checks for anything left behind in that process, far more sensitive than any other test or machine can do.

If it wasn’t for horseshoe crab blood, creating medicine that is safe for injection would be a lot harder and potentially more dangerous.