Why do universities place a weight on GRE/TOEFL scores?
GRE/TOEFL scores are used in a number of different ways (some of which are alluded to above)


GRE scores are sometimes used as a university-level filter (if your GRE score is &lt; X, then you'll need a strong support letter from your department to get admitted)
TOEFL scores are used as a filter for giving people TA funding (if your TOEFL score is too low, you can't be assigned to be a TA, and if it's lower, you can't even get RA funding)
More informally, the GRE/TOEFL scores are used as a "do you even care' filter: for a CS program, a quantitative score less than 600 might be considered to be a warning that the candidate doesn't even care enough to prep for it. 


But for Ph.D programs, the GRE/TOEFL are either used as a high-bar disqualification filters to prune applications (in top schools), or as low-bar disqualification filters to prune the non-serious applications.