No one can predict the PR accurately as Google is always updating and chaning the algo used for computing the PR. Also, a number of factors are considered for calculating the PR and only G knows the exact weightage assigned to each factor.
PR Predictions just for the fun of it. Of course it exist just as predicting the weather existing, though predicting the weather nowadays are far more accurate than predicting PR.
PR predictions are not accurate, but some do show you backlinks and use their own algorythm to give you some idea. It can be fun