We all used to talk about what players are better than others, and many times it came down to, "This guy is a little better hitter, while that guy is a little better fielder, baserunner, or whatever...." or "one guy has more power, while the other walks more." It was always hard to quantify the different aspects of the game.
WAR attempts to answer those questions, but it is no debate ender. They do attempt to take out the "subjectivity" as much as possible and base their values on scientific research that tries to quantify how much each event leads to a scored or allowed.
I think they do a pretty good job, but there are flaws and some major variances between bWAR and fWAR that make us shake our heads and ask just how accurate they are.
I like to look at this simple way: compile several lists of players by season, last 3 years, 5 years, 10 years using BA, HRs, Fldg %, OBP, SLG, OPS, wRC+ or whatever single stat or data point you wish and compare it to the top WAR players and ask yourself, which single list best orders the players by overall value?
I think the answer is pretty clear, to me.