What are the benefits of water/liquid fasts?

No calories for days: forces people into ketosis, i.e. burning fat, thus body fat stores reduce (like what happens to people in concentration camps; not fed or fed minimal). However, many, in response to fasting (with lots of cheap carb foods widely available) end up storing back more fat (after stopping the fast) than they lost during the fast. Thus generally, in modern USA culture, often ends up a net negative.

Uncertain: Various health conscious individuals advocate periodic fasting as a means of"detoxifying" the body. Western medicine doesn't support the practice. Fasting for weight loss 1-2 days a week can be effective but most individuals would find this difficult to do. Fasting for spiritual reasons may be of psychological benefit for some.