As a beginner, a strap acts as an extension of your limbs to help you achieve the benefits of poses and build memory for proper posture.
As a yogi who has had time away from practice, or someone who is recovering from an injury, a yoga strap will aid you in easing back into poses and help you to better execute poses without compromising your alignment.
For advanced yogis a strap is a great tool to challenge yourself through creating resistance in a pose to deepen your practice whilst maintaining proper alignment.
Or for many of us out there, maybe you have one leg longer than the other, or short arms or whatever unique quality it may be, yoga straps can help to fit a pose to your body.