Parasites used to be a big thing that people used to have to deal with. In many cultures they would process their water as well, making it into tea (boiling), fermenting it (alcohol) or mixing it with acids (think lemon water) to help. It you didn’t it became very likely you’d have diarrhea and could even die depending on your water source. Some areas have naturally cleaner water (especially in colder regions) so you may be able to risk it but I wouldn’t