Tapered shirts are dress shirts are designed with a slimmer fit through the torso and waist while maintaining a comfortable shoulder and chest measurement.
Why tapered mogs regular fit
The tapered shirts are going to mog "regular fit" shirts. "Regular fit" shirts can turn out looking boxy, where as Tapered shirts are tailored to follow the natural contours of the body for a sharper look. This will make your shoulder width to waist ratio look favourable in clothes.
You can see on the right, the non tapered look looks scruffy, appears to have a 1:1 shoulder width as the shirt isn't following his natural body contours, and it just does not have aesthetic look that something like the tapered shirt is going to give you.