The first characteristic that draws players to polys is their durability. Strings generally snap because they grind against each other during play. Polyesters are less abrasive than other strings, resulting in less wear and tear. Gustavo Kuerten's polyester-strung French Open run in 1997 set off a trend that's become the norm. 

The texture and stiffness of polyesters also allow the strings to better bite the ball and apply additional spin. Plus, polys actually move down at contact and snap back into place while the ball is still on the strings, which enhances spin.

Additionally, many players find stiffer strings, like polys, better for controlling the ball. With less flex, the ball spends less time on the string bed, promoting a more consistent response—the
analogy being the difference between throwing a ball against a concrete wall versus a pitchback.

The caveat with many of the stiffer polys is you need adequate racquet-head speed to enjoy the benefits. That’s not an issue for pros, but it’s a definite concern at the rec level. Otherwise the strings won’t move at all, and it will feel like you’re hitting the ball with a rigid, wooden board. (For this reason, it’s recommended that polys be strung 5 to 10 pounds lower than nylon or gut.)