Win Percantage is calculated from WAA and playing time which is innings pitched for pitchers, plate appearances for hitters. Players frequently move between leagues in minors limiting their playing time making Rank/WAA less useful for evaluations.
Showing Win Percentage allows you to see the rate at which players gained or lost WAA. A rate of 0.500 is even steven, completely average.
Win Percentages are not applicable in MLB for ranking purposes so XXX fills that field in mlb records. A value of NA as a Win Percentage indicates not enough playing time to make that calculation