By creating stats where every value has a different payoff, you can create stats where every player wants a specific different stat for any given build.
Or, if using randomized stat values, every value creates a different build possibility.
If you get a character with a 14 strength, it's basically a given you'll use a medium-heavy weapon like a longsword.
To take advantage of the stat you have.
Whereas with a 13, maybe you'll take advantage of the extra inventory capacity...