Sforno on Numbers 15:39/ספורנו על במדבר ט״ו:ל״ט

1'וראיתם אותו וזכרתם את כל מצות ה, you will be reminded that you are G’d’s servants whose commandments you have accepted reinforced by oaths known as אלה and שבועה. This reminder will be due to your looking at the “fringes” that may be viewed as if their king had placed a stamp on your bodies confirming that the wearer is one of his subjects.

2This in turn will lead to your ceasing to follow the inclination of your hearts and eyes to indulge your diverse urges, originating in your bodies. Without these fringes as a constant reminder of your purpose on earth, you would be likely to fall prey to these urges inspired by the evil urge.

3This would eventually diminish the influence of your mind over your body to such an extent that it would lead to your premature death both in this world and in the hereafter.