Telling someone, on paper, that their role is X, tends to cause that person to conform to the role of X. And it tends to cause other players to expect them to behave like X, and assume they will behave like X, and causes those other players to adjust how they behave based on that assumption. This impacts ones ability to freely role play. The fighter isn't supposed to strike, they're supposed to defend. The wizard isn't supposed to defend, they're supposed to control. And so on. So behaviors tend to conform to expectations of the label. This is true in all of life, and particularly when it's spelled out in black and white by specific rules. It's not that you must behave that way - it's that more often than not, that's just how people react to roles being defined for them.
It's also why my freest feeling role playing has been with the lightest rules I've played.