When a and b are independent, P(AB) = P(A)P(B),

At this time, there is no statistical relationship between A and B. Naturally, P(B|A) =P(AB)/P(A) = P(B).

Many problems in reality, A and B are not independent of each other. At this time, P(AB)≠P(A)P(B), P(B|A) are different from P(AB) and P(B), so they need to be calculated according to their respective relationships and formulas.